(FORTUNE Magazine) – Economically the U.S. has had a near-perfect run under Bill Clinton. But in January, a new man will move into the White House. Al Gore and George W. Bush have promised to keep the good times rolling. How will they do it? Both will rely on a key aide named Larry.

Meet the two Larrys: Lawrence H. Summers and Lawrence B. Lindsey. Summers is the current Treasury Secretary and the economic ideas man behind Gore. He's expected to remain in his job if Gore becomes President, and would be the dominant voice on economic policy. Lindsey, a resident scholar at the American Enterprise Institute, is Bush's chief economic advisor. Campaign insiders say he would become Treasury Secretary or chair of the Council of Economic Advisors (CEA) if Bush wins. Either way, Lindsey would be influential in designing and executing Bush's economic policies.

What's initially striking about Summers and Lindsey are their similarities. They're alike physically--the same age (45), the same height (about 6 feet), the same generous build. Both earned Ph.D.s in economics from Harvard (Summers in 1982, Lindsey in 1985). Both had the same thesis advisor, Martin Feldstein, a leading conservative economist who chaired the CEA in the first Reagan Administration. Both got a taste of policymaking working for Feldstein at the CEA. In the broad spectrum of economic thought, both are mainstream. Both even began political life as Democrats. Then their paths diverged, and the choices they made along the way--in academia and in their careers--led them to very different conclusions about the kind of economic policy the U.S. should pursue in the early 21st century.

Summers was born and raised a Democrat. Like many economists of his generation, he was drawn to the field partly by the example of John Maynard Keynes, the British giant who revolutionized economics and preached that it could be a powerful force for social good. At Harvard, Summers was known for interests that ranged widely, from stock market volatility to economic reform of Eastern Europe. He set out to make his mark in academia, and at 28 became Harvard's youngest-ever tenured professor. In 1987 the National Science Foundation awarded him $500,000 to do economic research--the first such grant given to a social scientist. In 1993 he won the ultraprestigious John Bates Clark Medal, given to the most outstanding economist under 40. "He's done groundbreaking work that's changed the way economists like me think about things," says Robert Hall, who taught Summers as an undergraduate at MIT and is now at Stanford.

Summers became a leader of the New Keynesians, a band of young economists committed to reinvigorating the ideas and ideals of Keynes. Today he personifies the mainstream of American economic thinking: That markets work best when left alone but that government intervention--in the form of monetary and fiscal policy--is warranted, especially during downturns. At times Summers is prepared to go further and use government policy to soften the harsher edges of capitalism. Last year, for example, he was behind the Clinton Administration's decision to forgive the official debts of many of the world's poorest countries.

Lindsey, who bolted to the GOP as an undergraduate at Bowdoin, became fascinated with taxation at Harvard, where he studied how changes in marginal rates affect economic activity. He continued his research at the CEA. "Lindsey has done a lot of empirical research to understand how the world and the economy work," says Feldstein, "and then he's tried to apply it."

After his stint in the White House, Lindsey returned to Harvard. He completed his thesis on taxation in 1985 and remained at Harvard as an associate professor for four years. In 1990 he published his first book, The Growth Experiment: How the New Tax Policy Is Transforming the U.S. Economy, where he demonstrated that cutting taxes resulted in a much smaller loss of revenue than was widely believed. For instance, instead of the $44 billion revenue loss that was predicted to occur when the top marginal tax rate declined from 70% to 50% in 1982, Lindsey calculated that the true revenue cost was only $6.1 billion, in part because people worked harder when they knew they could keep more of each dollar. Lindsey's argument was the most compelling case for Reagan's tax cuts, and it came at a time when many economists had turned against supply-side thinking.

The term "supply side" is fraught with controversy. Under Reagan, it was a revolution focused on cutting marginal tax rates. Supply-side troops led by New York Rep. Jack Kemp--and inspired by California economist Arthur Laffer--carried the day, and between 1982 and 1988, the top individual tax rate fell from 70% to 28%. As the economy picked up, the U.S. tax cuts were widely emulated by other countries. More recently, supply side has been reduced to comic proportions by extremists like Steve Forbes and the editors of the Wall Street Journal, who seem to think tax rates are the only thing that determines economic performance. Few respectable conservatives, Lindsey among them, now want to be called supply-siders. But here is where Summers and Lindsey diverge. Lindsey believes strongly in the efficacy of tax cuts and would reduce them now. Summers believes this article of supply-side faith is overblown and that cutting taxes now would be wrong.

Summers became active in politics in 1988, as an advisor to Democratic presidential candidate Michael Dukakis. In 1991 he left Harvard for the World Bank, where he was appointed chief economist at the behest of the Bush Administration. In Washington things didn't always fall into place for the golden boy of economics. Summers made a big gaffe when he signed off on a semifacetious World Bank memo advocating that waste and pollution from the U.S. and other First World countries be sent to developing nations because they were "underpolluted." Many people--notably Gore, then a Senator from Tennessee and a rabid environmentalist--were appalled by the memo, which was leaked. When Clinton and Gore won election in 1992, Summers was passed over for the prominent job he had hoped for: chair of the CEA. He settled instead for the fourth-ranking spot at the Treasury Department, Under Secretary for International Affairs.

Summers became infamous in Washington for his brains and his lip. His intellectual arrogance did not play well in Beltway salons. Almost every Washington regular has a Summers story. One goes like this: Summers, as Deputy Treasury Secretary, was holding a meeting with a group of world financial leaders. Among those present were five former Japanese deputy finance ministers. Summers went around the room telling each why his policies had failed. "One by one Summers destroyed them intellectually," says a finance leader who was present at the meeting and asked not to be named. "You could feel the tension in the room rising."

To his credit, Summers set about to become a nicer--or at least a more diplomatic--guy. Phil Gramm, the influential GOP Senator from Texas, now describes him as "a worthy adversary. We always work well together." Summers' mentor, former Treasury Secretary Robert Rubin, says he knew it would be just a matter of time until Summers proved adept at personal politics. "Larry was actually very attuned to the issue of dealing with people," says Rubin. "At times he might have done it less effectively than he could have, and as a consequence it was something he was constantly working on." Summers' temperament is a touchy subject that he'd prefer not to discuss. "I've found that identifying mutual interests is a much more important means of persuasion than force of argument," he told FORTUNE. "I've tried to adapt that without losing rigor or thoroughness of thought."

Summers tried especially hard to mend his relationship with Gore. The two collaborated on the Administration's Russia policy in 1993-95. And Summers made a point of keeping Gore briefed during the Asian financial crisis in 1997 and the collapse of Long-Term Capital Management in 1998. Summers has also built a close relationship with Fed Chairman Alan Greenspan during years of working together, particularly in times of crisis, when they depended on each other's judgment. As for the big shoes Summers had to wear when he replaced Rubin, he seems to have filled them. "He stepped into the job 100% trained, and after three or four months official Washington forgot Rubin," says Robert Litan, an economist at the Brookings Institution and former director of the Office of Management and Budget.

Politicians and economists on the left and right still criticize Summers. In 1994 conservatives argued that large loans he made to prop up the Mexican peso would never be repaid. (They were.) Democrats were unhappy with the Rubin/Summers strategy of pursuing fiscal responsibility above all else and opposing social spending that had considerable support in liberal circles. (That tactic, too, seems to have worked.) During the 1997 Asian financial crisis, the left attacked Summers for supporting IMF prescriptions of free capital flows and restrictive economic policies.

Some of his fiercest critics include the leading Democratic academics of his generation, namely Joseph Stiglitz, former World Bank chief economist; Harvard professor Jeffrey Sachs; and Paul Krugman, a professor of international economics who recently moved from MIT to Princeton. Those who oppose Summers do so at their peril. The abrupt resignation of Stiglitz from the World Bank in December is a case in point. The many accounts of what happened have one thing in common: Summers thought Stiglitz had criticized him and the Administration too often and too publicly. People close to the principals say Summers insisted that Stiglitz go. A Treasury spokeswoman denied it, and Stiglitz had no comment.

While Summers pursued an academic career in the late 1980s, Lindsey left one to put his theories into practice. He first went to work in the White House as a special assistant to President Bush on policy development. In 1991, Bush chose Lindsey as one of seven presidential appointees to the Board of Governors of the Federal Reserve System. The Democratic Congress viewed Lindsey's appointment as highly controversial and held it up for nine months. But once he was approved, Lindsey became an active contributor to policy debates. "I found him extremely easy to work with," says Susan Phillips, a former Fed governor who is now dean of the B-school at George Washington University. "People liked him even if they didn't agree with him."

During his six years at the Fed, Lindsey worked regularly with Greenspan and today counts him as one of two mentors (the other is Feldstein). "I learned a lot at the Fed," says Lindsey. "Part of what I do is talk to people all the time. I talk to Greenspan and Feldstein, and I'm constantly bouncing ideas off them." On occasion, Greenspan has gone out of his way to help Lindsey. The Fed chairman, who never does on-the-record interviews, agreed to be one of four leaders interviewed for Lindsey's recent book, Economic PuppetMasters: Lessons From the Halls of Power.

Some of Lindsey's Fed colleagues (who asked to be nameless) say that he was occasionally too political in an organization that should be apolitical. "His comments often came out of nowhere," says a former colleague. "That's fine on a committee of 19 people but not if you're the main policymaker." Still, Lindsey was a decision-maker during what is considered to be one of the most successful decades of monetary policy in the Fed's history.

More recently Lindsey gathered leading conservative economists, including Feldstein and Stanford's John Taylor, to create Bush's $483 billion tax cut. They would phase in the cuts over five years and reduce the current five-rate structure to four. The lowest individual rate would be 10% (vs. 15% now), the highest 33% (vs. 39.6%). In January, Lindsey rallied the GOP faithful with a Wall Street Journal column arguing that the strong economy is due to Reagan's tax cuts, not to Clinton's deficit-reduction policies. It is this ability to formulate and articulate economic alternatives to Gore that makes Lindsey such a valuable asset to the Republicans.

How would the two Larrys advise their respective bosses? Perhaps their biggest macroeconomic difference involves the budget surplus. Summers would use it to pay down debt; Lindsey would apply it to tax cuts. Assuming that the stock market hangs tough, that productivity growth remains high, and that the expansion continues, these different strategies would put the economy on very different tracks next year.

Under Summers, there's no reason to believe the near future would be much different from the recent past. He probably would not be averse to additional spending on social programs, but he's hardly a liberal spendthrift. Summers supported Clinton's deficit reduction, which relied heavily on a tax increase. Republicans argued vehemently that higher taxes would slow the economy by discouraging spending and investment. But today, most economists agree that higher taxes have underpinned the expansion. The increased revenue helped balance the budget, and the resulting fiscal discipline gave Greenspan the cover to lower interest rates more aggressively than he might have otherwise. The tax increase also further eroded the supply-side argument that only lower taxes can produce a strong economy. Now Summers sees the surplus as an opportunity to lock in benefits from the boom--low interest rates, strong business investment, and high productivity--and also as a way to ensure the solvency of Social Security. So debt reduction would continue to be a priority.

Summers' thinking generates criticism from all sides, but much of it is about values, not economics. Liberals believe the country has an obligation in good times to help the less fortunate, so they feel the surplus should be used--at least in part--on social spending. The right argues that the surplus belongs to taxpayers and that it should be returned to them in the form of tax cuts. Summers may be sympathetic to the first criticism, but he would argue that ensuring continued growth is the best way to help the poor. As for the Republicans' tax cut proposal, he just doesn't buy it.

Ultimately Lindsey's tax cuts might generate greater rewards than Summers' debt reduction--but only if things go according to plan. With official forecasts predicting an accumulated surplus of $1 trillion by 2009, Lindsey believes money can go back to taxpayers without endangering the surplus. The benefits, he says, would be higher productivity growth and business investment, which would power the economy even more forcefully.

Lindsey's critics say tax cuts would cause the economy to overheat and lead the Fed to raise interest rates even faster than it has now. The economy would get hotter still if consumers decided to spend their extra income instead of saving it, and the growth would be unbalanced: Consumer spending would accelerate quickly, while business investment retreated. The prospect of higher interest rates doesn't faze Lindsey. He expects the benefits of lower taxes and faster productivity growth to offset higher rates. Essentially Lindsey would change the policy mix. Rather than tight fiscal policy and loose monetary policy, which has been the case most recently, he would prefer the opposite.

And what if the surplus fails to materialize? For Summers the chance to preserve Social Security would vanish. Lindsey would face a bigger dilemma: the choice between large deficits or the reversal of his tax cuts. With so much at stake, in this debate, it's hard to find a neutral voice. Perhaps the most authoritative is Greenspan's. He has said many times in congressional testimony that his first preference is to use the surplus to reduce the debt. But if that isn't an option--which it presumably would not be if Bush is elected--his second preference would be to cut taxes. Greenspan, in other words, can live with both Larrys.

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