TO ANYONE learning English for the first time, the meaning of "negative campaigning" ought to be clear enough. It is the opposite, surely, of campaigning. It is what you do if you have declined the chance to inform and persuade. The negative campaign, you might presume, is the dark, dismal, antithesis to actual politics.

No responsible candidate ever abdicates his or her responsibilities in such a fashion, of course. Never happens. All serious politicians - and they are always serious - deplore the negative campaign. Some say it is demeaning. Some call it simply dishonest. They all agree that it is a very bad thing for democracy. Then the dirt begins to fly.

You "go negative" for one of three reasons. First, because you are losing and have nothing left in the locker. If the polls are any guide, such is John McCain's current position. Can't win on the wars; can't win on the economy; and can't, despite every effort, escape the shadow of the fantastically unpopular George Bush. So cast doubt (and then some) on the identity, character and probity of your opponent. Better still, for the sake of plausible deniability, have others do the grubby work.

Hence the second reason for answering in the negative: you appear to be winning, but you can leave nothing to chance. If you are Barack Obama, this means refusing to be "swift-boated". This refers to the job done on John Kerry and his Vietnam record - complete with fake Jane Fonda photograph - by those who organised the so-called Swift Boat Veterans on Bush's behalf.

Democrats have a traditional failing in that regard. They walk into the sucker punches. They submit to the smears. They seek, small comfort though it turns out to be, to "rise above" the half truths and lies. Not Obama. His campaign has made an art, first, of instant rebuttal, secondly in keeping a stock of scandalous tales for retaliatory use. If the Republicans wish to play dirty, the Democrats are this time ready to oblige.

There is, though, a third reason for the negative campaign: what have you got to lose? It works. Research, experience and results have demonstrated the truth time and again. Voters respond. An electorate that pays pious homage to "the issues" is forever eager for a piece of scandal, especially when it confirms an existing prejudice. As Bill Clinton discovered, it takes something more than mere charisma, to dispel the fog of innuendo and falsehood.

Threats to national security will do it, if the candidate is a Republican. People will trust anyone who appears capable of keeping them safe. For a Democrat, the economy, good or bad, is the magic bullet. Obama has begun to establish clear leads even in the swing states because he seems capable of dealing with a crisis made in Wall Street and fostered by Bush while the economically-illiterate McCain stumbles. So the Republican, for equal and opposite reasons, talks character, and asks plaintively who "that one", Obama, "really is".

So far, so predictable. If you listen to the spinners and the "consultants", McCain's real error has been in leaving it too late, with only Wednesday's final presidential debate remaining, in adopting negative tactics. Or rather, in allowing Sarah Palin, the glove puppet aiming for the vice-presidency, to stage her sweetly-vicious attacks on Obama for "palling around with a domestic terrorist", keeping company with a dodgy property dealer and an indiscreet pastor, and in somehow failing to subscribe to the values of a "real" American.

Last week, Obama's people were answering in kind, more or less, by digging up a 20-year-old episode in which McCain was censured for his association with an individual convicted over a savings and loan scandal. The fact that the Republican has been heard to sing "Bomb, bomb, bomb Iran" to a Beach Boys tune was a bonus, a sick joke turned effortlessly against the joker.

But why should any voter care about such rubbish? That Obama has been in the same room as a professor who was active, 40 long years ago, in the ineffectual (but "terrorist") Weather Underground hardly counts as a hanging offence. The fact that Palin has problems with the truth is a matter of record, but hardly a political novelty. McCain has lots of cars and lots of houses. Vietnam hero or not, he is no Joe Sixpack. This does not, of itself, disqualify him from the presidency.

Yet dirt sticks. It sticks better than policy detail. It adheres more easily than the memory of a fine speech or a solemn promise. In an odd way, candidates are humanised by smears even as they are demonised. The slippery concept of "character", misrepresented or not, still matters. And the professionals exploit it ruthlessly.

The irony is that the "real" Obama is probably as truly elusive as the real McCain. It is an illusion, a media conjuring trick, that persuades the electorate that somehow it knows these people. But an election is a choice. Let's say you are a conservative by conviction and upbringing. Against your will, you might just have begun to believe that Obama is the man able to pull America out of an economic hole. But the candidate's middle name is Hussein. You are informed that the man's a Muslim. His wife has said some odd things about the patriotism you hold dear and his erstwhile pastor seems to hate your country.

What's your choice? The cool, analytic intelligence capable of turning the economy around? Or the dislike inspired by fear and prejudice? Who has your ear?

A lifelong working-class Democrat might have similar problems. He might know that McCain is Bush by other means. He might have a son doing God knows what, for God knows which reason, in Afghanistan. He might work out that McCain's tax plans will favour the rich, as usual. So does he choose the charismatic big city elitist instead? Or does he swallow his reservations and pick the Vietnam vet who promises to find $300 billion to aid those, like our voter, whose home is about to be repossessed?

At minimum, negative campaigning sows such doubts. It distracts attention. It makes a big deal of details and obscures the larger picture. Last week Obama dismissed McCain's $300 billion plan as yet another scheme to line Wall Street pockets. Obama was forced to waste time, his and the electorate's, by even discussing a slice of pie-in-the-sky. When campaigns go negative, everyone dons blinkers.

The underlying reality is that McCain is in a bind. His is the party of George Bush. Therefore anything he might say about the economic catastrophe - and the candidate had little enough to say to begin with - will shackle him tighter to its chief author. With Bush's approval ratings at 25%, that's certain death. So McCain avoids the disaster whenever he can while his campaign hammers away at "character".

In another campaign, in a different year, it might have worked. Obama has not acquired substance overnight. He still speaks in platitudes. But yet again, in a long career of missed chances, McCain's timing is off. The primary contender who once faced the hellish accusation that he had fathered an illegitimate child by an unnamed black woman - back then, the Bush people denied all responsibility - can't even co-ordinate the smears. And he, of course, knows nothing of such things.

Very soon now, someone will nominate this as the dirtiest campaign ever. Since the last one, that is, and before the next. It is as though the process of selecting an American president creates a kind of vacuum, sucking in dirt. Yet, despite it all, one atrocity remains to be enacted in the days before November 4. How desperate could the Republicans become with that talk of "real" Americans?

The polls say it could be worth a whole 6% of the vote, but no-one has dared to put it plainly. Still, have certain good folks noticed that Mr Obama is, in fact, a black man? And why would that count as a negative?