For Democrats, Tuesday night will be a nail-biter for reasons beyond Hillary Clinton and her quest to repel Donald Trump at the polls. They are also deeply anxious about the other very big prize they must also capture to become the viable party of government in Washington DC.

How much of the territory lost over recent years in the US Congress can they reclaim?

If the outlook seemed encouraging for Ms Clinton, the story is more nuanced when it comes to Capitol Hill. Making at least some inroads into the Republican majority in the House of Representatives is a near certainty, but probably not to the point of overturning it.

But Democrats have been pouring resources and energy into seizing control of the upper chamber, which will be crucial for a President Clinton if she is to successfully enact her agenda and have her nominees for top positions confirmed, especially when it comes to filling the seat that his now empty in the Supreme Court and others that may be vacated in the next four years.

It should have been doable. The Republicans must defend no fewer than 24 of the 100 Senate seats while the Democrats have had to fortify only 10. Moreover, many of those GOP seats are in states that are purple, with voters roughly split in their allegiances to the two parties.

Yet nothing is certain according to the final batch of surveys released on Monday. The Democrats may have a slightly better than even chance of making the net gain of four seats they will need to seize control of the chamber, but slight is the right word. A four-seat gain would leave both parties with 50 seats, but the sitting vice president has the tie-breaking vote.

The Upshot in The New York Times puts their chances of pulling it off at 53 per cent, hardly a comfortable margin. Nate Silver's FiveThirtyEight data site says it’s 50-50, while the poll of polls at RealClearPolitics has the Democrats picking up only three seats, one short of what they need.

The Republican are evincing confidence they will see off disaster. “We are going to keep the majority,” declared Arkansas Senator Tom Cotton, a vice chairman of the National Republican Senatorial Committee.

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Democrats have been banking on the toxic presence of Mr Trump at the top of the Republican ticket to help dent the chances of the most vulnerable sitting Republicans, notably Ron Johnson in Wisconsin, Kelly Ayotte in New Hampshire and Pat Toomey in Pennsylvania.

But the presidential race appears to have tightened in recent days. And Republicans hope that the Trump effect, if it is negative, has been overstated. “I don’t think the presidential race will be a major factor … [voters] distinguish between the presidential race on the one hand … and the Senate races,” Senator Cotton added. It will also have helped that the Senate Leadership Fund has stoked their candidates’ chances with $37m (£30m) spent on adverts in the most vulnerable seats.

Florida is among the contested seats where the Republicans have confidence, thanks in part to the superior name-recognition and deeper pockets of their candidate, Senator Marco Rubio, who also ran for the presidential nomination this year before dropping out. His Democrat rival, Patrick Murphy, is shown several points behind going into voting on Tuesday.

As ever, Florida is likely to be a thriller on Tuesday night in the presidential race, with Ms Clinton possibly having the slightest of edges over Mr Trump. If she indeed prevails in the state, it is likely to be thanks in part to voters splitting allegiances on their ballots, marking her name for president and Mr Rubio’s for senator.

“If you don’t think elections in Florida can be close, I encourage you to go to Google and look up 2000 presidential election Florida,” Mr Rubio told supporters in Pensacola at the weekend, in a reference to the Bush-Gore presidential race of that year that was only settled after it was determined that George W Bush had won the Sunshine State by just 537 votes.

The Republicans are increasingly bullish about the chances that former Senator Evan Bayh who is trying to win back his seat in Indiana can be seen off, which would be a missed opportunity for the Democrats who had thought he would sail to victory in the state.