Most of the time, coming in short of the bronze there would finish off a candidate. But not Mr. McCain, who is now said to be the front-runner for the Republican nomination. He leads some polls in New Hampshire, where independents will be able to vote for him in today’s GOP primary.

His major rival in New Hampshire, Mitt Romney, is thought to be deeply if not mortally wounded by finishing second in Iowa. The winner in Iowa, Mike Huckabee, is polling poorly in New Hampshire and has little money. Mr. McCain may win New Hampshire, attaining thereby the momentum to win later primaries and the nomination. Only Rudolph W. Giuliani and Super Tuesday raise questions about Mr. McCain’s drive to the nomination.

Mr. McCain is now likely to play the electability card. He will claim to be the only GOP candidate who can beat Sen. Hillary Rodham Clinton or Sen. Barack Obama. It’s fair to ask, therefore, whether Mr. McCain is likely to win the general election, assuming he is the GOP candidate. Mr. McCain, like any candidate, must unify his own party and then reach out to enough independents to win.

But Mr. McCain has been an agent of Republican disunity. He came to national prominence as a “maverick” Republican. That meant he championed causes and policies contrary to the preferences of one part or another of the Republican coalition. He was one of two GOP senators who voted against the 2001 Bush tax cuts, and one of three who opposed the 2003 reductions. He also co-authored legislation calling for extensive regulations to deal with global warming. In 2001, Mr. McCain, Sen. Edward M. Kennedy and Sen. John Edwards introduced a Patient’s Bill of Rights that included several mandates for health coverage.

Mr. McCain has opposed some government spending, but overall the senator has little respect for the free market and thus little appeal to economic conservatives, a core GOP constituency.

In the 2000 primaries, Mr. McCain attacked religious conservatives, suggesting their leaders were “agents of intolerance” who had no place in American politics. More generally, the senator sponsored the McCain-Feingold campaign finance law, which passed over the opposition of 80 percent of congressional Republicans. The law sought to restrict the political speech of groups at the heart of the Republican Party: the National Rifle Association, anti-abortion organizations and businesses. Mr. McCain tried to exclude these groups from participating in elections. Why should they support him in November?

A prominent McCain sympathizer, The New York Times’ David Brooks, predicts that a President McCain would change not only the nation but also the GOP. Mr. McCain’s changes are not hard to predict. People and groups that worked for the Reagan victories and the 1994 Republican return to power would find themselves exiled.

Mr. McCain styles himself a maverick whose appeal to independents and Democrats might make up the lost GOP votes. But independents and Democrats this year want change. Mr. McCain is about continuity, not change. He is at least as much a hawk on Iraq as President Bush. He has as much principled opposition to government spending as Mr. Bush, which is to say, none at all. He differs from the status quo only in his willingness to tax more. Voters are likely to see the truth: A McCain presidency would continue and exacerbate the trends of the last eight years.

A McCain candidacy and defeat would be a near-term disaster for the Republicans, but it might be good for the party in the long run. Mr. McCain would be the standard bearer of big-government conservatism: taxing and spending at home and crusading for democracy in foreign lands. His defeat in November would show beyond all doubt that such “conservatism” has been a political failure. He would thus, at long last, open a sorely needed debate about the principles of the Republican party.