Leading article: Both Mr Bush and the Democrats are on trial

This year's mid-term congressional elections are, we are told, shaping up to be the dirtiest and most expensive ever. And as the United States enters the last frenetic week of campaigning, the reasons become ever clearer. President Bush and his Republican Party have their backs against a wall. In such circumstances, the Republican electoral machine driven by Karl Rove, Mr Bush's Svengali from his Texas days, has a history of resorting to desperate measures: the party's ample war chest is emptied, and every method of manipulation, however dubious, is applied.

American pundits are already describing these elections in vintage terms, suggesting that the Republicans may not have been this endangered since Watergate. Certainly, all the issues they have campaigned on so successfully at every recent election have dissolved around them.

The national security argument, a sure-fire election winner since the terrorist attacks of 2001, has been undermined, if not completely discredited, by the disaster that is the war in Iraq. American voters can no longer be bamboozled into seeing the war through the prism of the World Trade towers and al-Qa'ida. And Mr Bush is no longer convincing when he argues that removing Saddam Hussein purged the world of the terrorist threat.

The loyalties of the religious right to this Republican administration are also being sorely tried. George Bush came to office promising to return honour and dignity to the White House in the wake of Bill Clinton's dalliance with Monica Lewinsky. But while Mr Bush's personal conduct may be without blemish, the same cannot be said of certain senior Republicans.

The resignation of a Florida senator over lewd emails sent to young members of the congressional staff came at a crucial point in this campaign, as did revelations that some of the senator's more senior colleagues knew of the allegations earlier and failed to act on them. The memoirs of a Bush adviser, who claims that the White House regarded the religious right with contempt, while cynically milking them for their votes, may also discourage some Southern voters from turning out this time around. Add the Abramoff affair, in which members of congress of both major parties, but especially Republicans, were implicated in taking money that amounted to bribes from free-spending lobbyists, and the Bush administration starts to look vulnerable.

Everything, though, is overshadowed by Iraq. Americans, of whatever party, do not like failure; nor do they like to see their country on the wrong side of a moral argument. A little late in the day, maybe, but they are coming to realise that they are watching both in Iraq. The latest polls suggest that the Republicans' relatively safe majorities in the House of Representatives and the Senate are both at risk. The polls, though, may be less predictive of the outcome than they seem. So thoroughly have congressional districts been gerrymandered that relatively few seats are true marginals. This does not mean that the Democrats cannot prevail, only that national percentages are not all they seem.

Late "surprises" cannot be excluded, even though the scare tactics used by the White House before the last two sets of elections may have lost much of their persuasive power. The Democrats, though, still have work to do. Hampered perhaps by their initial support for the Iraq war and still fearing the potency of the security card in Republican hands, they have shown nothing like the discipline and drive that should have made victory theirs. If they win nonetheless, it will be proof as much of Mr Bush's weakness, as of their own fitness to steer policy in the two years before they next try to reclaim the presidency.