Polls suggest Republicans will be punished for conservative views on rape and abortion and fail to overturn 53-47 deficit
Democrats are increasingly confident of retaining control of the Senate – achieving a target that looked all but beyond them earlier this year – with polls suggesting the Republicans will be punished for their conservative views on rape and abortion.
The Republicans had been expecting to overturn the Democrats' slim 53-47 majority, adding the Senate to the already Republican-controlled House, providing them with a powerful base to challenge Barack Obama or support a Mitt Romney presidency.
But these hopes have receded. One of the biggest blows is in Indiana, a state the Republicans have held since 1976. The latest poll shows the Republican Richard Mourdock trailing 11 points behind Democrat Joe Donnelly. The Tea Party-backed Mourdock is suffering from his comment that pregnancy from rape was "something God intended".
A senior Republican, former Mississippi governor Haley Barbour, admitted on Sunday that the Grand Old Party was unlikely to take the Senate. Interviewed on CNN, he predicted it would be close. Pushed on what he meant by this, he went further: "Means no cigar."
Polls suggest that Congress will emerge from Tuesday's elections almost unchanged. The prospect of status quo means Washington faces at least another two years of deadlock, with the Democratic Senate and Republican House cancelling each other out and the chances of significant new legislation poor.
It will also mean that if Obama is re-elected, more standoffs between the White House and the Republican-led House, beginning with the impending deadline for dealing with the deficit and massive spending cuts – the so-called 'fiscal cliff'.
The Democratic leader in the Senate, Harry Reid, warned Romney last week that if he became president, he could expect little in the way of co-operation from the Democrats. He dismissed as "fantasy" his claim that he was better placed than Obama to bring back bipartisan co-operation.
Thirty-three of the senate's 100 seats are being fought over, with at least 10 of them regarded as toss-ups. All 435 House seats are also being contested.
The Republicans hold 241 seats in the House to the Democrats' 194. The Republican majority could be reduced but the chance of the Democrats retaking control remains extremely remote.
The races have produced a colourful array of candidates and offer the prospect of a number of firsts, from the first – at least to be open about it – lesbian senator, Tammy Baldwin, in Wisconsin, to the first Republican black congresswoman, Mia Love. There are minor historical footnotes, too, such as the almost certain return of a Kennedy to Congress: Joseph P Kennedy, the grandson of Robert Kennedy.
The Republican party has been in self-destruct mode in recent months, facing the prospect of failing in states that should have been easy gains, as it did in the 2010 mid-term elections when it fielded a disastrous set of Tea Party candidates.
As well as the prospect of losing an Indiana Senate seat, the Republicans have made it difficult for themselves in what was the Democrats' most vulnerable seat, Missouri. Republican Todd Akin's suggestion that some rapes were "legitimate" turned a near-certain gain into a toss-up race.
The Democrat Claire McCaskill had been struggling in Missouri, trailing Akin up until that remark. According to Real Clear Politics, she now holds a five-point lead. Republicans, after initially distancing themselves from Akin over the rape remark, are pouring money in behind him and polls show him eating into McCaskill's poll lead.
With a moderate in place instead of a Tea Party-backed one, the Republicans would have been almost guaranteed victory instead of sweating over the outcome.
The Republicans are almost certain to lose Maine, again as a result of the party's move to the right. It was held for the party by Olympia Snowe, an old-fashioned Republican moderate who believed in bipartisanship and became dismayed with polarised politics and the resulting Washington deadlock.
An independent, Angus King, is in line to take Maine and is expected to caucus alongside the Democrats. The Republicans ended up in the bizarre position of spending money on ads supporting the Democratic candidate in Maine, hoping to split the Democratic and independent vote, allowing the Republican to slip through.
Crossroads GPS, a super-political action committee co-founded by George W Bush's strategist Karl Rove, has spent almost $1m (£620,000) in attacks on King.
The Democrats are hopeful of taking back the normally solidly Democratic state of Massachusetts where Republican Scott Brown is battling to hold on against a strong push from one of the Democratic party's most prominent liberals, Elizabeth Warren. The Real Clear Politics average shows Warren ahead by five points.
Even if the Democrats were to lose their majority, they would still be able to block legislation from a Republican-controlled House. The Republicans would need a 60-40 majority in order to overcome any Democratic filibustering.
There is a small chance that when all the results are in on Wednesday that the Senate could be tied on 50-50. The casting vote would then rest with whoever is vice-president.
The key House races
Minnesota: A classic example of how fast political fortunes can change. ln August 2011, Michele Bachmann won the Iowa straw poll of Republican candidates, giving rise to a slew of media stories that she might be the party's presidential candidate. But that proved an illusion for the high-profile congresswoman, who is backed by the Tea Party. She is now battling to hold her seat in the House. A poll for the Minneapolis Star Tribune has her ahead in the polls but not enough for comfort, 51-45.
Utah: Mia Love could become the first Republican black congresswoman. Polls show her with a slight edge over the Democrat Jim Matheson.
Illinois: Tammy Duckworth, who lost two limbs as a helicopter pilot in Iraq, is the Democratic candidate battling to take a House seat from Republican Joe Walsh.
The key Senate races
Connecticut: One of the biggest-spending races in the country other than the White House. Linda McMahon, the Republican candidate who made a fortune as an executive promoting wrestling, is taking on Democrat Chris Murphy. She has spent more than $42m of her own money. But Connecticut is traditionally Democratic territory even if the previous incumbent Joe Lieberman, who is retiring, was an independent.
Missouri: One of the most widely watched races to see if Republican Todd Akin will be punished for his views on rape and abortion, summed up in his remark about "legitimate rape".
Massachusetts: Republican Scott Brown took normally solidly Democratic Massachusetts after the death of Ted Kennedy. The victory was a shock to the Democrats who hope that Elizabeth Warren can take it back.
Indiana: Repubican candidate Richard Mourdock, like Akin in Missouri, is on the defensive after explaining his opposition to abortion. "And I think even when life begins in that horrible situation of rape, that it is something that God intended to happen."
Wisconsin: Tammy Baldwin, a Democrat, could become the first lesbian – or at least the first to be open about it – elected to the Senate.