Just one term: what John McCain should pledge as he runs for president.

SEN. JOHN MCCAIN can increase his chances of winning the presidency in 2008 if he pledges not to run for it in 2012. He should pledge, that is, to serve only one term as president if elected. Why would any candidate do something so self-abnegating? Because it would help him both in the primaries and in the general election.

McCain was for a while the most popular politician in America, in large part because he wasn't seen as a typical politician. An admiring press corps helped put him in that flattering light. But he has lost both that reputation and the support of the press since 2004. Because he is taking the steps that are necessary to win the Republican nomination--such as calling for the retention of tax cuts he originally opposed---the press has fallen out of love with him and he risks appearing to be just another pandering, flip-flopping politician.

The number of McCain's flip-flops has been exaggerated. McCain has opposed a Federal Marriage Amendment because he doesn't think federal action on marriage is necessary. When McCain supported a marriage amendment in his own state, he wasn't being inconsistent on marriage amendments; he was being consistent on federalism. Many of his other moves can be similarly defended. But even if the perception of McCain is unfair, he needs to do something to change it.

To limit himself to one term as president would separate McCain from the rest of the pack---and particularly from Sen. Hillary Clinton, whose transparently calculating ambition is one of her major liabilities. McCain would implicitly be placing himself on the right side of the divide between those politicians who run to be someone and those who run to do something. In his case, presumably, that "something" would be to see America through an especially dangerous phase of the war on terrorism and, secondly, to address the nation's looming fiscal wreck. These tasks would be the veteran's last mission. The pledge would fit the persona.

Moreover, it would fit the narrative arc of McCain's career. He has been telling us for years that we should subordinate ourselves to a cause greater than self-interest. Giving up the possibility of a second term would be a way to show that he still means it. And if he runs on this twofold mission and wins, he'll have a clear mandate.

The pledge would also eliminate much of the rationale for a third-party campaign by New York City mayor Michael Bloomberg, which some Republican strategists think could hurt their party in 2008. Bloomberg needs the major candidates to stick to the usual scripts to give him the chance to present himself as fresh choice. McCain can make himself the fresh choice.

In 2008, the country will be reaching the end of two full, back-to-back eight-year presidencies for the first time since 1825. And both the Clinton and Bush presidencies have felt longer than their years. The country might be in the mood for something different, and four years of John McCain may well look more attractive than eight years of Hillary Clinton. During much of the past two presidencies, the country has been evenly divided between the parties and a kind of trench warfare has resulted. Each side has been fighting furiously over small strips of ground, always plotting the next maneuver. McCain has stood for political reform. A one-term pledge would let him embody it.