David Cameron faces a major by-election test as voters go to the polls in Corby, where the Conservatives are defending a slim majority.
A close fight is expected in the swing seat, which was won by the Tories in 2010 by fewer than 2,000 votes, for the first time since 1992.
Ed Miliband's Labour Party has established a solid poll lead since the last election and the Tories' campaign was plunged into controversy on Tuesday night when it emerged their campaign manager Chris Heaton-Harris had encouraged a rival candidate.
But Labour sought to play down expectations on Wednesday night, saying turnout would be very low because of the early dusk in November and forecasts for bad weather. "We think we have won the campaign but we are nervous about the outcome," a source said. "We will wait and see but whoever wins, no-one should expect it to be a big numerical majority."
The by-election, caused by the resignation of MP Louise Mensch to move to New York with her family, is one of three being held on Thursday. But those in Cardiff South and Manchester Central are not expected to produce an upset, each carrying a strong Labour majority from the last election.
Manchester Central, with a Labour majority of more than 10,000 from 2010, was vacated by Tony Lloyd so that he could stand in the Police and Crime Commissioner (PCC) elections. Cardiff Central, where Labour won by just over 4,700 two-and-a-half years ago, was prompted by the resignation of another PCC candidate, Alun Michael, who has held the seat for 25 years.
The Prime Minister is facing Labour calls to withdraw the Tory whip from MP Mr Heaton-Harris, who was forced to apologise after being recorded revealing that he suggested his friend James Delingpole, who writes for the Daily Telegraph on environmental issues, should stand in Corby on an anti-wind farm ticket.
Labour's vice-chair Michael Dugher said that the Conservative leader's failure to discipline the Daventry MP for a "serious betrayal of his party" stood in stark contrast to the decision to suspend the whip from Mid-Bedfordshire MP Nadine Dorries after she flew to Australia to appear in TV reality show I'm A Celebrity Get Me Out Of Here! without seeking permission.
In a statement, Mr Heaton-Harris said he had not "choreographed any sort of 'secret plan'". "It can hardly be said that my wanting to 'put this on the agenda' is new," he said. "I have spent years fighting onshore wind, getting over 100 MPs to sign a letter to the Prime Minister, securing debates and raising questions in Parliament and touring the country speaking to anti-wind groups."
But Mr Dugher said: "Even the Tory campaign manager doesn't think people should be supporting the Conservative candidate. If Chris Heaton-Harris doesn't have faith in the Tory candidate and his party, the public shouldn't." Home Secretary Theresa May, campaigning in Corby, said Mr Heaton-Harris had merely been "guilty of some silly bragging".