[an error occurred while processing this directive] Most polls are predicting small Republican gains, but this is no surprise given the historical trend for the party that holds the presidency to lose seats in mid-term elections.

Several polls showed no significant advantage for either party.

But with so many close races, one Democrat pollster suggested that "big things could be decided on the basis of very small victories"

The number of neck-and-neck contests means that the Republicans could capture anything from nine to 20 new seats in the House of Representatives.

In the Senate, the Republicans look unlikely to widen their lead on the Democrats by more than five seats, with some analysts saying that the net gain is likely to be only two or three. However, nine close races could make a big difference.

A poll by Associated Press suggests that most incumbents will be returned to office, as expected in a time of economic prosperity.

AP suggests that Democrats have abandoned their hopes of winning back the Republican-controlled Senate, but that Republicans are also resigned to making no more than small gains on their present lead.

Many voters have yet to make up their minds, according to a poll by the Pew Research Center which suggested that one in five voters is either undecided or expresses no more than a leaning towards a particular candidate.

Turnout crucial

The turnout is likely to sway the result, with the Democrats - who are more at risk from voter apathy than their Republican rivals - standing to gain the most if large numbers of Americans turn out to vote.

Democrats have also suggested that a last-minute Republican advertisement campaign which included veiled references to the Clinton sex scandal might have backfired, by shaking Democrat voters out of their apathy.

However close the race may be, commentators are suggesting that the outcome of the race will have an impact on the impeachment proceedings against Mr Clinton, with Republican gains likely to give the Republican congress members the confidence to push ahead quickly with impeachment.

All 435 seats in the House of Representatives will be up for re-election on Tuesday, along with 34 of the 100 seats in the Senate, and the governorships of 36 of the 50 states.