LONDON — Prime Minister Boris Johnson of Britain scored a striking political victory on Friday when his Conservative Party snatched a bellwether parliamentary seat from the opposition Labour Party, which had held it since the constituency’s creation in the 1970s.
In a by-election in Hartlepool, in the northeast of England, the Conservative candidate, Jill Mortimer, easily defeated her rivals, consolidating Mr. Johnson’s earlier successes in winning over voters in working-class areas that had traditionally sided mainly with Labour.
Better still for the prime minister, the vote on Thursday came after days of publicity over claims that he broke electoral rules over the financing of an expensive refurbishment of his apartment.
That appeared to have counted for little with voters in Hartlepool, an economically struggling coastal town, when the results were announced Friday morning after an overnight count.
Instead, voters may have been focused more on the gradual relaxation of Covid-19 restrictions in Britain after a successful vaccination program for which Mr. Johnson has been able to claim credit.
Though not unexpected, the outcome underscored the extent to which Mr. Johnson is rewriting Britain’s electoral map and dealt a blow to Keir Starmer, Labour’s leader. Mr. Starmer took over from Jeremy Corbyn last year after Labour’s defeat in the December 2019 general election, its worst performance in more than 80 years.
That landslide election victory for the Conservatives in 2019 followed the crisis over Britain’s exit from the European Union, and Mr. Johnson scored well in many traditional working-class communities with his appeal to voters to give him the power to “get Brexit done.”
Though Britain has now completed its European Union withdrawal, and the issue is fading somewhat, the new Conservative victory suggests that Mr. Johnson remains popular in areas — like Hartlepool — that voted for Brexit in a 2016 referendum.
Collectively known as the “red wall,” because they were once heartlands of the Labour Party, these areas are being targeted by Mr. Johnson who has promised to “level up” by bringing prosperity to the north and middle of England, and to areas that feel forgotten.
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.
Elections also took place on Thursday in Scotland and those could present a bigger threat to Mr. Johnson. Scotland’s first minister, Nicola Sturgeon, who leads the pro-independence Scottish National Party, is hoping for a strong performance that she can use to justify her call for a new referendum on whether Scotland should break away from the United Kingdom.