Leading article: The gloves come off

The tone of the United States Presidential election campaign has suddenly turned a good deal more personal. Last week saw the publication of allegations in the New York Times about the Republican front-runner John McCain and his relationship with a female lobbyist. There was also the storm over Michelle Obama's remark about feeling pride in her country for the first time in her "adult lifetime" because of the ground-swell of support for her husband in his campaign for the Democratic nomination.

And at the weekend, Hillary Clinton made a startling attack on Mr Obama for producing what she claims was a misleading leaflet on her own health-care policy. At a rally in Ohio, which will hold its primary next week, the New York senator scolded her rival with the words "Shame on you, Barack Obama". This has been interpreted as a decision by the Clinton camp to "go negative" on her opponent.

Of course there is legitimate argument over whether Ms Clinton's move really constitutes "negative" campaigning in the manner of the 2004 attacks on John Kerry's Vietnam war record, or the alleged smearing of John McCain in the 2000 Republican primaries by George Bush. Ms Clinton can quite plausibly claim she is taking exception to her opponent's misrepresentation of her policies, rather than anything more sinister. But one thing is pretty clear: all candidates are coming under greater pressure than they have hitherto felt.

The right-wing American "shock-jocks" seized on Ms Obama's remarks with the hunger of starving animals. And Mr McCain has responded to the New York Times report by demanding his supporters help to "counteract the liberal establishment". It is a pretty safe bet that a good deal of mud will be slung by the time the White House has a new occupant in January 2009.

But who will the targets be? On the Republican side it seems all but certain that Mr McCain will prevail, in spite of his recent difficulties. But the following nine days will be a critical phase in the Democrat race. Ms Clinton must either win convincingly in the states of Ohio and Texas or it is very likely that her campaign for the White House will be over. Even so, yesterday's announcement from Ralph Nader, the Green Party candidate who had such a dramatic impact eight years ago and who is considered by some Democrats to have cost them the White House, that he will stand again indicates that there could be still more drama in store.

This might or might not be remembered as the moment that the election campaign turned "nasty", but it does seem likely to be recalled as the point that things got deadly serious.