Britain’s Conservative Party suffered striking setbacks on Friday in local elections that are viewed as a barometer for how the party will perform in a coming general election and a key test for the embattled prime minister, Rishi Sunak.
Only a minority of the results had been announced by midday Friday, but already the signs were ominous for Mr. Sunak’s Conservatives. The party has lost about 150 seats so far, including six in Hartlepool, in northeast England, where it had made inroads after Brexit but has more recently lost ground to the resurgent Labour Party.
The Conservatives did score a notable victory in a closely watched race for mayor of Tees Valley, also in northeast England, where the Tory incumbent, Ben Houchen, held on, eking out a reduced majority.
Elsewhere, however, the picture was uniformly bleak for Mr. Sunak, under whose leadership the Tories have trailed the opposition Labour Party by double digits in national polls for 18 months.
In Blackpool South, a northern seaside district, Labour won a special election for a parliamentary seat in a huge swing of votes away from the Conservatives, who had held the seat but narrowly missed finishing third, behind Reform U.K., a small right-wing party. The previous Tory member of Parliament, Scott Benton, resigned in March after becoming embroiled in a lobbying scandal.
Labour’s leader, Keir Starmer, described the outcome in that seat as a “seismic win,” and the most important result of the day, though many more races were still to be declared.
“This is the one contest where voters had the chance to send a message to Rishi Sunak’s Conservatives directly,” Mr. Starmer said, “and that message is an overwhelming vote for change.”
Voters went to the polls on Thursday in 107 towns and cities in England to elect local council members as well as 11 mayors, including in London, the West Midlands and Tees Valley, in the northeast of England. More results will be announced throughout Friday and the weekend.
With Mr. Sunak’s party badly divided and time running out before he must call a general election by next January, the results were being closely scrutinized. While analysts expected the Conservatives to lose a significant number of seats, a worse-than-expected outcome could galvanize Mr. Sunak’s critics inside the party to try to topple him and install another leader.
The prime minister’s allies hope that some conspicuous victories — particularly in two regional mayoral races — would reassure Tory lawmakers, stabilize his shaky leadership, and end speculation about whether he will lead the party into the general election, expected in the fall.
Mr. Houchen’s victory in the first of those, in Tees Valley, eased some of the pressure on Mr. Sunak. But even that glimmer of good news was double-edged because Mr. Houchen campaigned mainly on his own brand, rather than that of his party affiliation, and his majority dropped from almost 73 percent of the vote in 2021, to around 53 percent.
The result of the other key mayoral contest, in the West Midlands, is not expected until Saturday and the Conservative candidate there, Andy Street, also distanced himself from the party during the campaign.
Even if the Conservatives win both those races, they are still braced for the loss of at least 400 council seats of the 985 they are defending. Many of these elections are in towns and cities that were traditionally dominated by the Labour Party, but that switched to the Conservatives in the years after the 2016 Brexit referendum.
To make matters more difficult, the last time many of these races were fought, in 2021, Mr. Sunak’s Conservatives were enjoying a period of popularity because of a robust rollout of a coronavirus vaccine by one of his predecessors, Boris Johnson. That means the Tories could have a long way to fall back.
In addition to Hartlepool, the Labour Party won control of councils in Redditch, Thurrock, and Rushmoor in Hampshire, although it had a setback in Oldham, where it remains the biggest party but lost overall control of the council after some of its seats fell to independents.
That result reflected dissent over Labour’s stance on the Israel-Hamas war, particularly among Muslim voters, many of whom believe the party’s leaders should be more vocal in criticizing Israel’s military action in Gaza.
“The idea that Labour is experiencing some localized difficulties relating to its stance on Gaza does seem to be playing out,” Chris Hopkins, political research director of Savanta, a polling and market research company, wrote in a commentary.
For Mr. Starmer, the election is a chance to show that he has a credible chance of becoming Britain’s next prime minister, as current polling suggests. Despite his party’s strong numbers, few voters seem enthused by Mr. Starmer, who is viewed as a competent but not especially charismatic politician.
Voters in London will have to wait until Saturday to discover if their mayor, Sadiq Khan, has won a third term, the first for a London mayor since the post was created in 2000. A defeat of Mr. Khan by his Tory opponent, Susan Hall, would be a big surprise, as the British capital leans left politically, but assuming he wins, the margin of victory will be watched for signs of dimming popularity.
The overriding challenge is for the Conservatives, however. A heavy loss of seats could demoralize the party faithful and panic Tory lawmakers, who fear that they will be tossed out of Parliament in the general election.
Since they were last elected in a landslide in 2019, the Conservatives have already ousted two leaders, Mr. Johnson and Liz Truss. Toppling a third would be risky, since there is no obvious replacement guaranteed to be more successful than Mr. Sunak, who was chancellor of the Exchequer under Mr. Johnson.
Mr. Johnson offered a reminder of his disorganized leadership style on Thursday when he turned up at a polling station without the necessary photo identification — a requirement that his own government introduced in 2022 — and was turned away (he returned later with the proper ID).
For Mr. Sunak, the polls are now so dire that some see a new leader as the only possible way to fend off a ruinous defeat in the general election. In January, one former cabinet minister, Simon Clarke, called on the prime minister to resign, but that failed to foment a bigger rebellion.
For those who think that such a gamble is worth taking, the aftermath of local elections would potentially be the last chance to move against Mr. Sunak before he puts his party on alert for the general election.
Dismal Conservative results in local elections in 2022 were a prelude to the ouster of Mr. Johnson, though not the trigger for it: He was finally purged two months later after mishandling an unrelated scandal.
Supporters of Mr. Sunak argue that he can benefit from a fall in inflation, the prospect of an economic recovery and by putting into action his plan to put asylum seekers on one-way flights to Rwanda, a policy that is popular among Conservatives and may help fend off a threat from the anti-immigration Reform U.K. party.