Before a Debate, the U.K. Election Campaign Just Got Messier

Prime Minister Rishi Sunak and Keir Starmer, the leader of the opposition Labour Party, will square off on Tuesday evening in their first debate of Britain’s general election. But it is a third man, Nigel Farage, who has seized the spotlight in a race defined, until now, by a fading incumbent and a rising opponent.

Mr. Farage, a gleeful insurgent who has long roamed the right-wing fringes of British politics, said he would run as a candidate for Reform U.K., a party he co-founded. That has shaken up the race and threatens to siphon off votes from Mr. Sunak’s Conservative Party, given Reform U.K.’s strident anti-immigration message.

Mr. Farage’s entry into the race is not by itself transformative. He has run for a seat in the British Parliament seven times — and lost every time. But his return could breathe momentum into other Reform U.K. candidates, throwing yet another hurdle into Mr. Sunak’s path between now and the vote on July 4.

The prime minister is struggling to avert a landslide defeat to Labour, which has held a double-digit polling lead over the Conservatives for more than a year. His debate with Mr. Starmer, though early in the campaign, already looms as a make-or-break chance to change a fast-congealing narrative.

“The election is over; it is done; Labour have won the election,” Mr. Farage said in declaring his candidacy in a surprise announcement on Monday. Describing it as “the dullest, most boring general election campaign we have ever seen in our lives,” Mr. Farage, 60, said the race needed “gingering up,” and offered himself as the tonic.

Mr. Sunak called the election on May 22, several months earlier than expected, in part to exploit a few glimmers of good economic news. He has moved aggressively to appeal to voters who might be attracted to the hard-right Reform U.K, proposing a national service requirement for 18-year-olds and floating a new law that would bar transgender women from women’s restrooms and female-only prisons.

But the Tories stumbled out of the gate on immigration when Mr. Sunak said his government’s flagship plan to put asylum seekers on one-way flights to Rwanda would not begin before the election. The Labour Party has vowed to shelve the policy if it comes into power, suggesting that the flights may never happen.

There is no evidence that Mr. Sunak’s decision to go to the voters early changed the dismal electoral picture for the Tories. A poll released Monday by the market research firm YouGov, which surveyed almost 60,000 adults, projects that the party will lose 225 seats while Labour will gain 220.

Though on the more bullish side of projections for Labour, those numbers would give the party a bigger majority than even that won by former Prime Minister Tony Blair in his landslide victory in 1997. The poll does not project that Reform U.K. will win any seats, a testament to the obstacles that smaller parties face in winning seats in Britain’s first-past-the-post electoral system, though it was conducted before Mr. Farage’s announcement.

For Mr. Farage, analysts said, the decision to run for Parliament may be part of a grander strategy to take over the Conservative Party after its expected defeat. But throwing his hat into the ring now is not without risks, they said, and they go beyond his own potential eighth consecutive defeat.

“On the one hand, it grabs the headlines and will almost certainly prove yet another nail in the government’s coffin,” said Tim Bale, a professor of politics at Queen Mary University of London. “On the other, if he does the Tories too much damage, those Tory M.P.s left in Parliament, and even some of the party’s grass-roots activists who purport to love him, aren’t going to feel too warmly toward him.”

“Still,” Professor Bale added, “a hostile takeover is still a takeover.”

Whether or not he wins, Mr. Farage will electrify a campaign that got off to a soggy start, going back to Mr. Sunak’s announcement, made in a drenching shower outside 10 Downing Street.

While Mr. Sunak retreated from Rwanda, Mr. Starmer’s Labour Party lost several days to an internal squabble over Diane Abbott, a Black member of Parliament who was suspended from Labour last year for suggesting that Irish, Jewish and Traveler people did not face racism in the same way that Black people did. (Travelers are nomadic minority groups that are among the most disadvantaged in Britain.)

Ms. Abbott, a revered figure on the party’s left, had been expected to bow out of the election in return for having the suspension lifted and being given a peerage in the House of Lords. But after she balked and the party’s progressive wing rose up to defend her, Mr. Starmer said she was “free to go forward as a Labour candidate.”

Ms. Abbott, 70, confirmed that she planned to run to regain her seat in North London, putting an end to an episode that distracted from Labour’s theme of “change” after 14 years of Conservative government.

Mr. Starmer tried to regain his footing on Monday with a speech in which he pledged to increase Britain’s military spending and modernize its nuclear arsenal. He said he would not hesitate to use nuclear weapons to defend Britain, a statement designed to push back on the Conservative critique that Labour is weak on national security.

Conservative officials pointed out that the last time the country’s Trident nuclear-weapons system came up for renewal, in 2016, senior Labour figures, including David Lammy, the shadow foreign secretary, and Angela Rayner, the deputy leader, both voted against it. (Mr. Starmer voted to renew it.)

“This is a changed Labour Party, and the most important thing is, I voted in favor of a nuclear deterrent,” Mr. Starmer said. “I lead from the front; I’ve always led from the front.”

Given the size of Labour’s lead over the Conservatives, analysts said the biggest danger for Mr. Starmer was self-created problems, which could cause voters to have second thoughts about the party. That is why the dispute over Ms. Abbott’s status frustrated some Labour supporters.

But Mr. Starmer’s challenge pales next to that of Mr. Sunak, who is trying to claw his party back from oblivion. He has campaigned energetically but unevenly, laughing at wisecracks over his rain-sodden debut and gamely accepting umbrellas.

Not a natural politician, the prime minister has soldiered through campaign appearances and photo opportunities that have occasionally backfired. Last week, a young man, referring to the party’s proposal for compulsory national service, asked him, “Why do you hate young people so much?”

On Sunday, Mr. Sunak released a TikTok video to poke fun at what he said was the Labour Party’s lack of plans. He turned the cover of a flip chart to reveal a blank page. Within minutes, Labour operatives had tweaked the video to list the party’s goals on the blank page. The next day, Mr. Sunak was photographed chatting with residents of Henley-on-Thames, England. Behind him sailed a boat with supporters of the Liberal Democrats, cheering and waving signs.

Mr. Sunak has long ruled out any alliance between the Conservatives and Reform U.K. On Monday, he brushed aside the threat from Mr. Farage, who will run for a seat in the seaside constituency of Clacton.

“At the end of the day on July 5, one of two people will be prime minister, either Keir Starmer or me,” Mr. Sunak said to broadcasters. “A vote for anyone who is not a Conservative candidate is just a vote to put Keir Starmer in No. 10.”