Former MP Louise Mensch has said she would accept responsibility if the Tories lost the marginal seat of Corby.
Labour sources said they were "cautiously optimistic" of winning back the Northamptonshire seat from the Conservatives, with voter turnout at more than 40%.
The results of that by-election are not due to be announced until later on Thursday, by which time Labour are expected to have held their two safe seats of Manchester Central and Cardiff South and Penarth. In Wales, Labour sources are quietly confident they can hold onto the Cardiff seat.
Taking to Twitter, Mrs Mensch - who gave up the seat in August so that she could move to New York to be with her family - said she accepted responsibility if the Tories saw their slim majority of less than 2,000 evaporate.
She tweeted: "Election result will not be a verdict on either Christine, or the Conservatives, but only on the decision I took to step down mid-term." Mrs Mensch added: "Though I had to resign to unite our family, there was large & entirely understandable local anger. I wish I had been able to see out my term."
The by-election in Corby is regarded as the first major by-election test for Prime Minister David Cameron. The Tory candidate Christine Emmett is expected to lose out to Labour's Andy Sawford, who will need to overcome a Conservative majority of 1,951 votes.
Labour has established a solid poll lead in Corby since the last election and the Tories' efforts were plunged into controversy on Tuesday night when it emerged their campaign manager Chris Heaton-Harris had encouraged a rival candidate.
Neither the votes in Cardiff nor Manchester are 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 South and Penarth, where Labour won by just over 4,700 votes 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.