The battle for Birmingham Erdington looked set to go down to the wire as Labour claimed bad weather and low turnout meant the by-election race would be "really tight".
The vote in the West Midlands seat was triggered after well-liked Labour frontbencher Jack Dromey died suddenly in January.
If victorious, Labour's Paulette Hamilton will be the city's first black MP but she has faced a strong challenge from fellow councillor Robert Alden, who leads the Tory group on Birmingham City Council.
A Labour source told the Mirror: “The weather’s absolutely grim and turnout is looking extremely low. It’s going to be really tight.”
Mr Dromey held the seat at the 2019 general election - when Labour suffered its worst defeat since 1935 - with a majority of 3,601, but the constituency had the fifth lowest turnout in the country at 53.3%.
Activists were concerned that the seat's historical low turnout would be compounded by a day of rain.
One Labour MP campaigning on Wednesday said it was "honestly hard to say" what would happen. They added: "No love for the Tories, but turnout is very low and the weather is rotten. I’m not sure anyone could have a real sense of how it will play out."
Anything but a Labour victory would be seen as a major upset for Keir Starmer's party, given Boris Johnson's Tories plummeted in the polls in the wake of the Partygate scandal.
The Conservatives came close to winning Erdington in the past. They lost by just 231 votes in 1983, when leader Margaret Thatcher was at the peak of her powers after the Falklands War.
One Tory source told the Mirror: "We are expecting a respectable showing. "People are very unconvinced with Starmer. Some outright hostility."
Labour had bused in activists from other parts of the country and general secretary David Evans was personally briefing teams before they headed out to doorstep voters, one activist said.
Ms Hamilton, a former nurse and mum-of-five, is close to victory, but became embroiled in row after controversial comments she made in 2015 about postal voting and "an uprising" were unearthed by GB News.
She was recorded as saying: “The postal vote is the way some of our ethnic minority communities actually lock it down. Because what they do is whole families just have the postal vote.
“Whoever comes in says hold this one, tick, postal vote goes back, and in one household we can have eight votes and it’s locked down.”
She also said: “You talk about the bullet or the vote, I’m not sure, although I believe in the vote, and I believe in our right to use that vote or destroy that vote, I’m not sure that we will get what we really deserve in this country using the vote.
“But I don’t know if we are a strong enough group to get what we want to get if we have an uprising. I think that we will be quashed in such a way that we would lose a generation of our young people. So I am very torn.”
Labour backed its candidate, however, saying: “Paulette Hamilton is arguing for better representation for the black community in public life, and as she is campaigning to become Birmingham's first black MP she has a point."
The comments are likely to come under further scrutiny in the days ahead.
Elsewhere, tackling crime, health inequalities, anti-social behaviour and housing shortages have dominated the campaign.
Ms Hamilton also faces competition in the strongly pro-Brexit seat from Reform's Jack Brookes, Lib Dem Lee Dargue, a number of independents and the Green Party's Siobhan Harper-Nunes. There are 12 candidates in total.
The family of Mr Dromey, a shadow minister married to Labour grandee Harriet Harman, said he died suddenly at the age of 73 in his flat in the constituency in January.
The former leading trade unionist had represented Birmingham Erdington since 2010.
