Votes are counted in the by-election triggered by the death of Labor MP Jack Dromey.
Labor held Birmingham Erdington in 2019 with a majority of 3,601 votes and anything but a win for Sir Keir Starmer’s party would be a blow to his leadership.
Paulette Hamilton is running for the Labor seat, while fellow town councilor Robert Alden is vying to become the first Tory MP since 1936.
Both sides sought to manage expectations when counting the votes.
Campaign teams worked into the final hours of the contest on Thursday to try to boost their side’s turnout on a day when bad weather likely did not encourage casual voters to come to the polls.
Low turnout could increase the chances of an upset, while candidates from smaller parties could erode the Labor vote.
A Labor source said: ‘The weather is absolutely gloomy and turnout appears to be extremely low. It’s going to be really tight. »
A Tory source said Boris Johnson’s party expected a “respectable performance”.
The Tories have come close in the past when they lost by 231 votes in 1983, when leader Margaret Thatcher was at the height of her powers after the Falklands War.
Mr Johnson’s party struggled in national polls as the Prime Minister was beaten by allegations that Downing Street parties broke coronavirus rules.
Mr Dromey, a shadow minister married to Labor great Harriet Harman, died aged 73 in his constituency flat in January.
The former leading trade unionist has represented Birmingham Erdington since 2010.