Leading article: A shadow over the end of Bush's presidency

Americans vote today in mid-term congressional elections that can be described as potentially the most significant since 1994. They have the capacity to change the priorities of US politics almost overnight, to consign the Republican Party to at least two years of opposition, and to blight the last two years of George Bush's presidency. An enormous amount is at stake.

This has been a rip-roaring coast-to-coast campaign of the sort that vindicates America's claims to be one of the world's most vibrant democracies. It culminated in news of the death sentence on Saddam Hussein, an all-American preacher scandal, and the predicted narrowing of the polls. It also illustrated how profoundly the mood in the United States has changed.

Two years ago, Mr Bush was able to campaign virtually unchallenged on the imperative of the "war on terror". The primacy of national security was a given. The Democrats' weakness - aside from the political tin ear of their presidential candidate - was that their every challenge could be parried with the charge that they were unsound on security. One thousand more dead American soldiers later, Mr Bush finds his anti-terrorism credentials called into question. A stream of inquiries and books has exposed flaws in the President's self-justifying arguments. Mr Bush's desperate ratings now make him one of the least popular US presidents on record. Vulnerable to the charge that the Iraq war - "his" war - is fuelling terrorism, rather than eradicating it, he has been shunned by Republican candidates seeking re-election.

Ethics scandals - over the activities of the lobbyist Jack Abramoff, and the sexually explicit e-mails of the Florida Congressman Mark Foley - have weakened another plank of the Republicans' campaign, their claim to moral superiority. And while the economy may be performing better than many forecast, Bill Clinton's famous dictum, "It's the economy, stupid", has for once been disproved. Whatever the scale of Republican losses tonight, it was not the economy; it was the war.

No one, though, should take anything for granted. However welcome a change of Congressional power would be, the elections are not over until the last votes have been counted. We learnt the lesson in 2000 and again, less dramatically, in 2004. This time around it is hard to gauge, for instance, what effect, if any, the conviction of Saddam Hussein will have: while it keeps Iraq in the news, it may help to convince Republican waverers that the invasion of Iraq did achieve something.

The longer-term consequences of any result must also be borne in mind. If the Democrats win control of the House of Representatives, but not the Senate, Washington is in for two years of stalemate. If they win control of both Houses - a harder proposition and thus a greater achievement - Democrats will be cock-a-hoop, and there will be much soul-searching among Republicans as they contemplate the presidential race in two years' time. Mr Bush will be written off as even more of a lame duck.

But there are hidden snares here for the Democrats, too. Yes, a resounding victory would cheer the party and those around the world who hope for a shift away from what has been a disastrous US foreign policy. Mr Bush, though, is an adroit politician who might set out to work with a Democratic Congress as effectively as Mr Clinton worked with the Republicans after the rout of 1994. And a share of power might turn out to be more of a hindrance than a help to the Democrats in their ambition to regain the White House. The more impressive the Democrats' showing tonight, the more firmly they must keep their feet on the ground. They - and we - must be careful what we wish for.