In fact, Labour would probably have already lost the Hartlepool seat in the 2019 general election had the Brexit Party, then led by Nigel Farage, not put up a candidate to run there and won more than 10,000 votes, pulling pro-Brexit voters away from the Conservatives.
The Labour Party lawmaker elected in Hartlepool then, Mike Hill, resigned his seat in Parliament in March because he faces an employment tribunal relating to sexual-harassment accusations, which he denies. His departure prompted Thursday’s vote.
Labour’s defeat in Hartlepool could intensify attacks from the left of the party on Mr. Starmer although, with no obvious alternative leader in sight, he is unlikely to face serious difficulties.
The pandemic, and the focus on the vaccine drive, has made it hard for the Labour leader to raise his profile, but critics say he lacks charisma and a compelling political vision.
And the loss of Hartlepool will be keenly felt by Labour given that it had been held by the party since the current constituency was created in 1974. Among those who have represented the seat are Peter Mandelson, a close ally of the former prime minister, Tony Blair.
Moreover Mr. Starmer knows that, if he is ever to become prime minister, he needs to rebuild support in the north of England and in the Midlands.
On Thursday voters cast ballots in local elections in many of these key areas and many results are expected on Friday.