The most expensive - and arguably the nastiest - mid-term election in US history shuddered through its final hours today after an avalanche of last-minute spending and amid a blizzard of "dirty tricks" claims against both sides.

With Republican control of Congress and the prospect of hobbling President Bush’s last two years in the White House at stake, there was anecdotal evidence from key states such as Virginia and Ohio that turnout could be the highest in decades for a Congressional election.

Mr Bush voted at sunrise in Texas, where he issued a plea to Americans of all political persuasions to exercise their democratic rights. "We live in a free society and our government is only as good as the willingness of our people to participate," he said.

But even as he spoke, problems with electronic voting machines, allegations of ballot rigging and claims of intimidation began.

The protests were loudest in the most closely-contested states and most of the noise was coming from Democrats, many of whom are convinced the last two presidential elections were stolen from them.

The FBI said they were investigating complaints that voters in Virginia had received a telephone call from someone pretending to work for the Democrats giving them misleading information about where they should vote. There were similar allegations in New Mexico.

In Ohio, all 12 voting machines in a predominately black district in Cleveland crashed when voting opened at 6.30am, feeding fears that minority - and therefore largely Democrat voters - were being disenfranchised.

Election officials in Indiana went to court to get a three hour voting extension after a computer glitch in 70 precincts delayed poll opening.

Colorado Democrats were opposed by the Republicans when they asked a state judge to allow late voting, arguing that similar problems had forced long queues to wait up to two hours - while others were turned away.

Electronic voting machines were also reported to be malfunctioning in Cook County, Illinois and Lebanon, Pennsylvania. In Madison, Wisconsin, a bomb threat closed a polling station.

Florida’s Broward County, notorious for the "hanging chad" debacle in the disputed 2000 presidential race, had more problems today as 14 voting machines in one precinct stopped working for 90 minutes.

In Maryland, Democrat officials accused their opponents of launching a "conscious, premeditated" campaign of voter intimidation after it emerged that Republican poll watchers had been issued with a handbook urging them to challenge voters who they suspected of not having the correct paperwork with them.

But in New Jersey it was the Republicans claiming to be victims of a dirty tricks operation with their candidate, Tom Kean, alleging that his campaign headquarters had been padlocked by Democrat operatives. His opponent, Bob Menendez, claimed it was a sleazy Republican stunt.

Such name-calling reflected the bitterness with which campaigns across America have been fought in the past two months.

According to the independent Centre for Responsive Politics, at least $2.8 billion will have been spent by both parties and their allied groups, 20 per cent higher than 2002, itself a record year.

Much of this money has been spent on negative attack adverts, which have darkened TV screens ever more in the past week.

More money has been spent by both parties on intensive get-out-the-vote operations by both parties, which could be critical in midterm elections when turnout is usually low, typically around 40 per cent.

The Democrats need to make net gains of 15 seats in the 435-member House of Representatives to seize control for the first time since 1994. To re-take the Senate, they must win at least six seats from 33 being contested today.

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