(FORTUNE Magazine) – While most of my best friends are not Republicans, some of them are. So it is hardly news that my good friend and former student Greg Mankiw will be voting for George W. Bush. Still, it's a bit disconcerting to see a brilliant economist supporting a candidate who derides the use of arithmetic as "fuzzy math." I'll vote for Al Gore, thank you--for many reasons.

Start with Social Security. Governor Bush is trying to sell the electorate a Social Security "plan" that is mostly blank spaces. (Is that fuzzy policy?) He wants voters to jump from the fact that the stock market normally outperforms Treasuries (which is true) to the conclusion that the transition to privatization will be easy (which is not true). Greg points out one problem: The Bush camp should not compare a 6% real return on private investing with a 2% real return inside Social Security. The low 2% figure is pulled down by the burden of what we owe current retirees, while the higher figure is not. Bad math!

But the fuzziness goes much deeper. Virtually every coherent privatization plan requires either significant new sources of tax revenue or sharp cuts in benefits (or both). Many also need huge loans that put the Social Security Trust Fund into the red for decades. None of these options are politically palatable, but actuarial calculations demand some combination of them to balance the books. Which does Mr. Bush favor?

Vice President Gore's plan for Social Security is explicit-- and a stark contrast. He opposes privatization as both risky and inimical to the interests of the less well-to-do. He would leave Social Security's tax and benefit structure more or less intact and would prevent the Trust Fund from falling into debt. Instead, he proposes to start filling the financial hole by transferring large amounts of general revenue to the Trust Fund--revenue we are likely to have only if we maintain fiscal discipline. And he would keep private investing outside of Social Security--in new "Retirement Saving Plus" accounts.

The Bush team's math is fuzzy also when it comes to the budget and taxes. The governor's huge tax cut amounts to $1.3 trillion over ten years--only if you count just nine years, ignore debt service, and forget about that old bugaboo, the alternative minimum tax, which prevents high earners from taking certain deductions and tax benefits. Add these in, and the true cost is closer to $2.2 trillion. Even if you accept Mr. Bush's budget numbers without question, including a zero price tag for missile defense, the amount of debt reduction over ten years is less than half of what Mr. Gore proposes.

Greg is right that the Gore tax proposal is complicated. But it is also far more fiscally prudent (under $500 billion over ten years) and vastly more progressive than the Bush proposal. Indeed, the complexity of Mr. Gore's tax cut stems mainly from the desire to limit its overall size and target the benefits to those who need them most.

Most readers of FORTUNE won't need to consult the accountants Greg warns of, because virtually none of the benefits of Gore's plan go to families with incomes exceeding $100,000. As for the hoops the under-$100K set would have to jump through to qualify for tax cuts, they would have to finance education, pay for health insurance, care for close relatives, save for retirement, or be married. These don't seem onerous or idiosyncratic to me.

There may indeed be a difference in how the two camps would approach Microsoft and other antitrust cases. Teddy Roosevelt notwithstanding, Democrats and Republicans don't always agree on how aggressively to go after alleged monopolists. Vice President Gore does not believe a President should tell the Justice Department how to enforce the law. But he does believe the antitrust laws have contributed to the spectacular competitiveness of American industry for over a century.

Finally, there is the matter of school vouchers, which Greg favors even though, in his words, they "could end public education as we know it." Al Gore does not look kindly upon the prospective demolition of the public school system. That's a real difference. But regardless of your personal view on school vouchers, remember that the critical decisions will be made at the state and local levels, not in Washington.

These issues illustrate a general pattern. On the broad questions of budget discipline and fiscal responsibility, America has been spectacularly successful in the past eight years. Al Gore vows to continue that stellar performance, while George W. Bush elevates a large tax cut to top priority and presents a dubious budget full of fuzzy math. In terms of inequality, the record is much spottier: Income disparities now stand near historic highs. Al Gore's plan bends over backward to help people in the middle and below and to shield them from peril. And George Bush? Well, you know.

ALAN BLINDER is a professor of economics at Princeton University and an outside advisor to Al Gore's campaign.