A smear campaign of the ugliest sort is now coursing through the contest for the presidency in 2000. Using the code word "temper," a group of Senate Republicans, and at least some outriders of the George W. Bush campaign, are spreading the word that John McCain is unstable. The subtext, also suggested in this whispering campaign, is that he returned from 5 1/2 years as a POW in North Vietnam with a loose screw. And it is bruited about that he shouldn't be entrusted with nuclear weapons.

There has of course been rumor-mongering in previous presidential campaigns: that so-and-so is cheating on his wife, that someone's wife or husband has shady relations or business practices. In 1988 the Republicans (including President Reagan) fanned a rumor that Michael Dukakis had mental problems, but that collapsed quickly of its own weightlessness. But nothing as nasty as what's being rumored about McCain has occurred in a very long time.

Among the members of a small group of Republican senators, and sometimes their staff, participating in this whispering campaign are Majority Leader Trent Lott; Majority Whip Don Nickles; Paul Coverdell, secretary of the Senate Republican Conference; and Utah Sen. Robert Bennett. They have let it be known that they won't come forward and say these things openly, out of deference to McCain. How thoughtful of them. (This will of course be met with denials.)

The Bush campaign has told reporters that it has heard that McCain's temper is a real problem, and that they're trying to find out more, and may use the issue. This is hardly a hands-off approach.

In fact, a core of Senate Republicans are known by their colleagues to hate--the word their colleagues use--McCain. He doesn't always observe the punctilities of Capitol Hill. He calls things as he sees them; he tells people off when he thinks he's been crossed; he points to pork-barrel projects in appropriations and other bills and names names. As a senator friendly to McCain--and there are several--put it, "He gets in their face."

One day last year, when the Republican Conference, at its regular Tuesday lunch, was discussing the bill to settle the tobacco wars, which had been shepherded by McCain through his committee 19 to 1 (only to be killed by the tobacco lobby), Sen. Pete Domenici displayed a complex and mocking chart. McCain stood up, said, "I think that's chicken ----," and sat down. Domenici was not amused.

There's no question that McCain does pop off. A friend of mine who was the object of a McCain tirade described it as a most unpleasant experience. But if a bad temper is a disqualifier from the Oval Office, then a lot of people--including several past occupants of that office as well as the current one--haven't been qualified. And in 1992, there were numerous stories about George W. Bush telling off, in very strong terms, campaign aides who he thought weren't doing enough for his father's reelection. President Clinton, we know, indulges himself in screaming at his aides.

To suggest that his experience in Vietnam prisoner-of-war camps left McCain mentally unbalanced is a useful device for undermining one of his most admired traits: his heroism and sacrifice in the service of his country. McCain sometimes makes himself hard to be liked, even by senators outside the group that is smearing him. "He's difficult to work with," says one sometimes ally. "Even when he's right, he can come off sanctimonious." This senator went on, "I find him both very admirable and very exasperating." He also said, "The whispering campaign that he's unstable is totally unfair. It's just payback."

A number of McCain's colleagues have noticed that the whispering campaign began just as McCain started taking off in New Hampshire, substantially narrowing the gap between himself and Bush. So, the more important question is, were McCain to be elected president, could he govern? (This is, of course, always a "compared to whom" matter, which forces questions about his opponents' capabilities and faults.)

One of the most important questions is whether a potential president could lead. One way to try to ascertain this is to attempt to separate real enthusiasm from fake, manufactured enthusiasm on the part of the people who observe him. Another is to try to discern why people support a given candidate: because they're genuinely excited by him (or her) or because they think he'll win. (Or they like his parents.) Is the candidate attracting a following or gathering votes?

Ironically, McCain is currently hot in New England because of the very traits that bug some of his colleagues: his directness, his calling it as he sees it. If--and we're a long way from there yet--he were elected, he would be a phenomenon and likely have the people with him, which could change the equation in his relationship with certain folks on Capitol Hill. And despite his unpopularity in some quarters on Capitol Hill, he has a respectable string of legislative successes.

Anyway, does a small group of senators get to block someone from office because they dislike him intensely? Or because they fear that someone might gain power who doesn't think the world of them? If they really think someone is so seriously flawed that he should not hold the highest office, that he presents a danger to the nation, should they not come forward and say so openly, and let the people hear an open debate on the matter?

The writer's most recent book is "The Corruption of American Politics; What Went Wrong and Why."