By Mark Landler and Stephen Castle

LONDON — Britain’s Conservative Party suffered crushing defeats in the race for what had previously been two safe seats in Parliament, but narrowly avoided losing a third contest, in election results Friday (21) that rendered an ominous verdict on Prime Minister Rishi Sunak’s political future.

The main opposition Labour Party won its biggest by-election victory since 1945 in Selby and Ainsty, in Yorkshire in the north of England, while the centrist Liberal Democrats scored a thumping win in another former Conservative stronghold, Somerton and Frome, in the southwest of England.

But the Conservatives avoided a sweep by narrowly holding on to Uxbridge and South Ruislip, in the northwestern fringes of London, a district that had been represented by the former prime minister, Boris Johnson.

For Sunak, who has been weighed down by a cost-of-living crisis and scandals involving Johnson and other Tories, the victory in Uxbridge was likely an outlier — driven by an unpopular plan by London’s Labour mayor, Sadiq Khan, to extend a costly low-emission zone to encompass the district.

The other two races, analysts said, are a better gauge of Britain’s anti-incumbent mood after 13 years of Conservative rule and provided a possible preview of the general election that Sunak must call by January 2025.

Voters in reliably Tory bastions of Britain’s north and south came out strongly against the Conservatives, suggesting that unless there was a significant shift in the political landscape in the coming months, the Tories are on track to lose to Labour in the next national election.

By-elections occur when a seat in the House of Commons becomes vacant between general elections. These races were triggered by the departure of Johnson, who was rebuked by his peers for misleading them over his attendance at lockdown-breaking parties; Nigel Adams, a close ally of Johnson’s in Selby and Ainsty; and David Warburton of Somerton and Frome, who admitted to using cocaine.

For Labour, the shift in Selby and Ainsty was one of its most dramatic in decades: its candidate, Keir Mather, overturned the largest Conservative majority in a by-election since 1945 with a 21% swing in the vote. At 25, Mather will become the youngest member of the House of Commons.

“This is a historic result that shows that people are looking at Labour and seeing a changed party that is focused entirely on the priorities of working people with an ambitious, practical plan to deliver,” the Labour leader, Keir Starmer, said.

For the Liberal Democrats, the victory in Somerton and Frome was numerically even more impressive: Sarah Dyke, a Somerset counsellor, overturned a Tory majority of more than 19,000 to win the seat by 11,008 votes.

Because the contests occurred in very different parts of England, they provided an unusual snapshot of public opinion before the general election. They also captured several trends that have run through British politics since the last general election in 2019, when Johnson’s Conservative won a landslide victory.

Uxbridge and South Ruislip is the sort of seat that Labour has needed to win to prove that it is credibly closing in on power. Its failure to do so was attributed by the victorious Conservative candidate, Tuckwell, to public anger toward the mayor of London, Sadiq Khan, a Labour member, for his plans to extend a costly ultralow emission zone across all of London’s boroughs, including Uxbridge.

While the result could raise some questions about Labour’s ability to win the next general election, the scale of the defeat in Somerton and Frome will most likely alarm Conservative lawmakers who are under pressure in some of the party’s heartland districts in the south of England.

The loss in Selby and Ainsty was also a big blow to the Tories in a very different part of the country, and will buoy Labour’s spirits.

With Britain besieged by high inflation, a stagnating economy and widespread labour unrest, his Conservatives face a real threat of being thrown out of power for the first time in 14 years.

While Britain shares some of these economic woes with other countries in the wake of the pandemic and Russia’s invasion of Ukraine, the Conservatives amplified the problems through policy missteps and political turmoil that peaked in the brief, stormy tenure of Sunak’s predecessor, Liz Truss.

She proposed sweeping but unfunded tax cuts that alarmed the financial markets and triggered her own downfall after only 44 days in office. Sunak shelved Truss’ trickle-down agenda and restored Britain’s fiscal stability. But her legacy has been a poisoned chalice for Sunak and his Tory compatriots with much of the British electorate.

“The Liz Truss episode really dented their reputation for economic competence, and that will be very hard to win back,” said Tim Bale, a professor of politics at Queen Mary University of London. “It’s going to be very difficult.”

So convincing is the Labour Party’s lead in opinion polls that some analysts predicted in advance that Sunak would become the first prime minister to lose three so-called by-elections in one day since 1968.

But the narrow victory for the Conservatives in Uxbridge and South Ruislip averted that prospect.

By-elections take place when a seat in the House of Commons becomes vacant between general elections. This time around, the contests were also a reminder of the toxic legacy of another of Sunak’s predecessors, Johnson.

Johnson resigned his seat in the district of Uxbridge and South Ruislip, on the western fringe of London, after lawmakers ruled that he lied to Parliament over lockdown-breaking parties held in Downing Street during the pandemic.

Voters in Selby and Ainsty in northern England were selecting a replacement for one of Johnson’s closest allies, Nigel Adams, who quit after not being given a seat in the House of Lords, as he had expected.

The contest in Somerton and Frome, a rural district in southwestern England, took place because another Conservative lawmaker, David Warburton, gave up his seat after admitting he had taken cocaine.

“This is probably the closing of a chapter of the story of Boris Johnson’s impact on British politics,” said Robert Hayward, a polling expert who also serves as a Conservative member of the House of Lords. But he added, “Whether it’s the closing of the whole book is another matter.”

In winning in Selby and Ainsty, a Tory stronghold, Labour hoped to show that it has regained the trust of voters in the north and middle of England — regions it once dominated but where it lost out to the Tories in the 2019 election.

The vote in Somerton and Frome was a test of the Conservative Party’s fortunes in its heartland areas of southern England, known as the “blue wall” — after the party’s campaign colours. It has been under pressure in the region from a revival of the smaller, centrist, Liberal Democrats.

The Liberal Democrats have benefited from some voters, who are opposed to the Conservatives, casting their ballots strategically for whoever seems best placed to defeat the Tory candidate.

Recent British elections have featured talk of a grand political realignment, with candidates emphasizing values and cultural issues. But analysts said these by-elections have been dominated by the cost-of-living crisis — kitchen-table concerns that hurt the Conservatives after more than a decade in power.

-New York Times