Republicans are slapping each other on the back this morning, figuring that the entry into the presidential race of perennial candidate Ralph Nader will cost Senator Kerry a percentage or two or three at the polls in November, making the re-election of President Bush that much more likely. That exuberance strikes us as a bit premature — and maybe even entirely wrong.

Sure, there’s a scenario under which Mr. Kerry has to run to the left on the issues to prevent liberal voters from defecting to Mr. Nader. That could cost him votes in the center, and cost him the election, as Vice President Gore learned in 2000. But there’s an alternative scenario, one that, as our Luiza Ch. Savage reports on page one of today’s New York Sun, is sketched by Mr. Nader’s nephew, Tarek Milleron, in an article for a left-wing Web site. Mr. Nader could run anti-Bush ads too “risky” — read, nasty — for Mr. Kerry or any mainstream Democratic group to touch.

Mr. Nader could create the illusion that Mr. Kerry is, in the relative sense, a defender of capitalism and a strong defense. If instead of echoing Messrs. Gore’s and Nader’s “people versus the powerful” rhetoric, Mr. Kerry chooses to refute Mr. Nader on the substance, then Mr. Kerry could yet emerge as a candidate palatable to mainstream America. It would be a kind of triangulation, with Mr. Kerry positioning himself as the sensible centrist alternative to Mr. Bush on the extreme right and Mr. Nader on the extreme left.

Some of that was already happening yesterday. As Mr. Nader was launching his campaign Web site that called for spending more on “schools, clinics, libraries, forests, parks” and less on “militarism,” Mr. Kerry was boasting on ABC News’s “This Week,” “I have voted for the largest defense budgets in the history of our country.”

Our sense of Mr. Kerry based on his record is that he is in fact a lot closer to the Nader left than to the centrist mainstream of American politics that is represented by Mr. Bush. But as Mr. Kerry arrives today in New York, one of the things he will have to think about is whether he wants to attract potential Nader voters or, instead, to take the risk of attacking Mr. Nader’s ideas head-on in the hope of attracting potential Bush voters.

Mr. Nader’s candidacy thus is both a potential opportunity and a potential danger for both Messrs. Kerry and Bush. While we differ sharply with Mr. Nader on much of the substance, he’s likely to enliven the campaign. If Mr. Kerry sees fit to include the Rev. Al Sharpton in the presidential primary debates, as he has so far, there’s no reason not to include Mr. Nader in the presidential debates of the general election. The smart thing for Mr. Bush would be to find a right-wing independent candidate of his own so it’s not a two-on-one debate and so that Mr. Bush, too, can find some points — immigration, his initiative to fight AIDS in Africa, prescription drug benefits — on which to distinguish himself from the fringe elements in his own party.