Votes are being counted in the by-election triggered by the death of Labour frontbencher Jack Dromey.
Labour held Birmingham Erdington in 2019 with a majority of 3,601 votes and anything but a victory for Sir Keir Starmer’s party would be a blow for his leadership.
Paulette Hamilton is contesting the seat for Labour, while her fellow city councillor Robert Alden is vying to become the first Tory MP there since 1936.
Both sides were seeking to manage expectations as votes were counted.
Campaign teams worked until the final hours of the contest on Thursday trying to boost their side’s turnout on a day where poor weather is unlikely to have encouraged casual voters to the polling station.
Low turnout could increase the chances of an upset, while candidates from smaller parties could erode the Labour vote.
A Labour source said: “The weather’s absolutely grim and turnout is looking extremely low. It’s going to be really tight.”
A Tory source said Boris Johnson’s party was expecting a “respectable showing”.
The Conservatives have come close in the past when they lost by 231 votes in 1983, when leader Margaret Thatcher was at the peak of her powers after the Falklands War.
Mr Johnson’s party has been struggling in the national polls as the Prime Minister was battered by allegations of parties in Downing Street breaching coronavirus rules.
Mr Dromey, a shadow minister married to Labour grandee Harriet Harman, died at the age of 73 in his flat in the constituency in January.
The former leading trade unionist had represented Birmingham Erdington since 2010.
