Many key races neck and neck as Republicans try to wrest more power from Democrats.

Democrats in the United States are bracing for punishment in tomorrow's midterm elections as voters express their dissatisfaction with President Barack Obama, who mostly stayed away from the campaign trail for fear of sinking his party's chances wherever races were close.

Yet it will be a nail-biting night for candidates all over the country running for seats in the US Senate, the House of Representatives and state-wide offices including governorships.

The Republicans hope to pick up six seats in the Senate to seize the majority from the Democrats and thus shatter the last bulwark protecting Obama's agenda in Congress.

But candidates in many key races that will decide the Senate majority remain razor-thin close.

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Now, with the last speeches done and the final TV spots cut, it's all about getting out the vote. Republicans at the weekend poured money into Alaska, Georgia and Iowa in a bid to make sure every last supporter who has not voted early shows up. Democrats were doing the same, galvanising their key blocs, including women and African Americans.

"There doesn't seem to be a lot of things for people to feel good about," said David Yepsen, director of the Paul Simon Public Policy Institute at Southern Illinois University. "It may not be fair, but they tend to take those kinds of feelings out on the White House, and as a practical matter I think the Senate goes Republican."

In the House, Republicans are expected to build on their majority of 233 seats to 199 for the Democrats. They are also likely to retain their majority in the number of governors' seats they hold in state capitals.

But the heavy campaign action has been in the 100-member Senate, where Republicans need to pick up six seats to reclaim the majority for the first time since the 2006 election. While Republicans are expected to gain seats, as many as eight to 10 Senate races are still considered toss-ups that could go either way.

There is a good chance the party that controls the Senate will not be known tomorrow. Senate races with multiple candidates in Louisiana and Georgia, where the winner must get more than 50 per cent of the vote, could be forced into run-offs in December or January, respectively.

If Republicans do take control of the Senate, Obama faces even more partisan gridlock in his last two years.

A Republican-led Senate would be likely to push ahead with approval of the Keystone XL crude oil pipeline, chip away at provisions in Obama's signature healthcare law, and move toward a broad rewrite of tax laws.

Democrats say the number of close races still gives them a shot.

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"The Republicans have made the President and his agenda the issue. The only way Democrats are going to win in the end is if they remind voters that all politics are local," said Democratic strategist Jim Manley.

Republican strategist Kevin Madden said: "We've seen the country lurch from crisis to crisis and confidence in the President and Washington as an institution has eroded."

But in the end, these midterms are about getting the best footing for the race to be the next President. Will Hillary Clinton run and what Republican can take her on?

Looking for numbers

House of Representatives:

Republicans:

233

Democrats:

199

Seats:

435, 3 vacancies, 2 independents

Control:

Republicans

Senate:
Republicans: 15 of 45 seats up for vote
Democrats: 21 of 53 seats up for vote
Seats: 100
Control: Democrats - Republicans need a net gain of 6 seats.

- Independent