(FORTUNE Magazine) – Larry Summers, Treasury Secretary designate, is probably the smartest person who has ever been up for the job. He certainly knows more about the way the economy works than any previous Secretary. So why is it likely--and it is likely, according to the drums along the Potomac--that his Senate confirmation hearing will turn contentious?

It's not as though there's any doubt he'll be confirmed--not with the President, the outgoing Treasury Secretary, the Federal Reserve Chairman, and even Trent Lott, the leader of the opposition party in the Senate, all behind him. And it's not as if anyone thinks he'll move economic policy in a new direction. The markets considered that possibility and rejected it in a few minutes on the day the news broke. Their vote--the collective opinion of the world's traders, investors, and seers--was crucial, and unambiguous: Summers will do just fine.

Summers' qualifications (and connections) are well known: youngest tenured professor in Harvard's 363-year history; winner of the hotshot John Bates Clark award as the best economist under age 40; son of noted economists; nephew of Nobel laureates (Kenneth Arrow and Paul Samuelson); deep thinker and capable lieutenant of the Clinton economic team. But Summers' best qualification is less obvious. More than anyone else on the policy stage, he embodies the consensus of modern, mainstream economics. It's a cautious consensus--suspicious of untested theory, respectful of markets, nonideological, humbled by its own failures to predict the future.

The controversy over Summers is of two kinds. The first comes from the scattering of people who quarrel with mainstream economics. They occupy the extreme ends of the policy spectrum. Those on the left tend to echo Harvard's Jeffrey Sachs, who argues that the mainstream approach to the crises in developing nations has caused unnecessary pain. He may be correct, but we'll never know. The others, on the right, are among the tiny but vocal remnant of the supply-side school. They warn that a Summers Treasury could endanger the world economy by imposing excessive fiscal and monetary discipline on developing nations. But the supply-siders have been so consistently wrong that they've forfeited their right to be taken seriously. A year ago they were warning that Treasury policy would worsen what used to be called the Asian economic crisis. Six years ago they predicted that policy would bring down the U.S. economy.

One way to think of these three points of view is to imagine that your body is the world economy, that the recent tumult in world markets is a life-threatening disease you've contracted, and that the various policy approaches are alternative courses of treatment. Summers and the mainstream represent the modern, big-city hospital; Sachs is the new, unknown drug that you have to go to Mexico to get; and the supply-siders are Christian Scientists. Who do you call?

The second source of controversy about Summers is a matter of style. In the academic milieu in which he grew up, the person with the better argument usually wins. In politics, by contrast, the better argument doesn't always win, and the best argument rarely does. It is compromise that carries the day, and though you may think your opponent a fool--and let him know it--you will nevertheless address him as "my good friend, the distinguished member from..."

Earlier in his Washington career, Summers tended to default to wise-guy-academic mode--and quickly created a bad impression in a town where first impressions are hard to change. But he has become steadily less argumentative and more courtly; his Rose Garden speech the day Clinton appointed him was a model of Beltway decorum, humble to the point of obsequiousness and generous to a fault.

As to whether he'll become a brilliant Treasury Secretary, if we're lucky, we won't find out. The U.S. economy is so strong that the current expansion will almost inevitably last into next year, making it the longest in history--an outcome Summers will be at pains not to disturb. And by this time next year, his tenure as Treasury Secretary will most likely have become a dress rehearsal for his next job: maybe Treasury Secretary in a new Administration, maybe chairman of the Fed. Maybe then we'll know how brilliant he really is.