With Keir Starmer, Britain and the U.S. Are Back in Sync, but for How Long?

It was a carefully staged display of big-power theatrics — or more precisely, of middle-power-meets-big-power theatrics.

“You are now connected to Air Force One,” said a White House operator, as Prime Minister Keir Starmer of Britain hunched over a speakerphone, in a short video released on Saturday by 10 Downing Street.

“Mr. Prime Minister, congratulations,” said President Biden, who was flying to Wisconsin for a campaign rally. “What a hell of a victory!”

The two leaders spoke warmly about the importance of the “special relationship” between Britain and the United States, their shared commitment to defending Ukraine and an upcoming rendezvous: Mr. Starmer is leaving on Tuesday for Washington, where Mr. Biden is hosting a NATO summit.

Not since Barack Obama’s first term have the White House and Downing Street both been in the hands of center-left parties. After years of tensions over Brexit, Northern Ireland and frustrated British hopes for a trade deal, this could augur a new era of harmony in the trans-Atlantic relationship.

It could also be a brief era. The American election in four months could restore to the presidency Donald J. Trump, who managed to have vexed relationships with a right-of-center British leader, Prime Minister Theresa May, let alone a left-of-center one.

Managing Britain’s relationship with one of its closest allies at a time of acute uncertainty will be a delicate challenge for Mr. Starmer, according to diplomats and analysts. He faces a Democratic president on the defensive, even within his own party, and the specter of a Republican replacement who would be at odds with him on core issues, from Ukraine to climate change.

The Labour Party’s landslide victory would normally be welcome news for Mr. Biden. British voters have a habit of presaging political change in the United States whether it was Margaret Thatcher’s election a year before Ronald Reagan’s, or the 2016 Brexit vote five months before Mr. Trump’s election.

But Mr. Starmer’s win, while thumping, came with caveats, not least the strong showing of an insurgent, anti-immigrant Reform U.K. party, led by Nigel Farage, a vocal ally of Mr. Trump’s. And Mr. Biden has his own problems, having to do with actuarial tables rather than political cycles.

“For this government, it’s all going to be about hedging against who’s going to be the Democratic candidate, hedging against whether Donald Trump is going to be elected, hedging against what U.S. policy is going to be, regardless of who is elected,” said Leslie Vinjamuri, the director of the U.S. and Americas program at Chatham House, the British research institution.

Mr. Starmer, she said, will even have to gauge issues like how to react to the results of the American election, especially if it is close and does not go in Mr. Trump’s favor. When Prime Minister Benjamin Netanyahu of Israel congratulated Mr. Biden on his 2020 win — a result that Mr. Trump continues, without evidence, to dispute — Mr. Trump reacted angrily and nursed a grudge against Mr. Netanyahu.

Given such scenarios, diplomats prefer to focus on what Mr. Starmer could accomplish with Mr. Biden over the next six and a half months. The two are united on issues like military support for Ukraine, aggressive government action to curb climate change and closer ties between Britain and the European Union.

The last one could really change the tone, analysts said, given that trans-Atlantic tensions over Brexit date back to before the Brexit referendum. Mr. Obama famously warned Britons that they would be at the “back of the queue” for a trade agreement if they voted to leave the European Union.

“The political relationship has been strained since the U.K.’s decision to quit the E.U., not least because of the risk Brexit posed to the smooth implementation of the Good Friday Agreement in Northern Ireland,” said David Manning, who was Britain’s last ambassador to Washington under a Labour prime minister, Tony Blair. The Good Friday Agreement was the 1998 accord that ended decades of sectarian violence in Northern Ireland.

“This is the chance for a new start,” he said.

Mr. Starmer will be accompanied to Washington by his foreign secretary, David Lammy, who has close ties to Mr. Obama through Harvard Law School, which they both attended. He and his boss do not harbor big hopes for a trade agreement, given the Biden administration’s lack of interest in such deals.

But Mr. Starmer could defuse lingering tensions over Northern Ireland, which got caught up in the often-hostile negotiations with Brussels over the terms of Britain’s departure from the E.U.

That issue rankled Mr. Biden, who proudly celebrates his Irish ancestry. He regularly warned previous British governments not to take actions that would jeopardize the Good Friday Agreement.

In his first visit to Belfast since becoming prime minister on Monday, Mr. Starmer described Labour’s victory as a reset for Northern Ireland and promised a “respectful and collaborative” relationship.

Mary Lou McDonald, the leader of Sinn Fein, the Irish nationalist party, said the difference in dealing with Labour and Conservative governments was like “daylight and dark.”

Even under Mr. Starmer’s predecessor, Rishi Sunak, tensions over Northern Ireland had eased somewhat. Mr. Sunak struck a deal last year with the European Union on the trade arrangements for the North.

A bigger source of tension, analysts said, could emerge over issues like commercial ties with China, where the Biden administration has pushed Britain and other allies to take a more combative approach.

“Britain needs economic growth,” Ms. Vinjamuri said. “Choosing between the U.S. and China is not a good position for the U.K. to be in.”

If Mr. Trump is elected to a second term, analysts said, he would not care about Northern Ireland, but he would be put off by Mr. Starmer’s efforts to draw closer to Europe. To the extent Mr. Trump had a warm relationship with any prime minister, it was with Boris Johnson, who gleefully clashed with the European Union and bore a cursory resemblance to the president’s own brand of populism.

That’s not to say that British and American leaders from opposite parties can’t work together. Mr. Obama, after all, issued his Brexit warning at the behest of David Cameron, a Conservative prime minister who called the referendum but campaigned against leaving the European Union. And Mr. Blair famously supported George W. Bush’s war in Iraq, with Mr. Bush even relying on him to consult with other leaders on his behalf.

“Bush did find it useful that he could take the temperature with Blair,” Mr. Manning said. “It’s hard to imagine Trump wanting that kind of relationship, but much depends on what kind of approach he would take to America’s traditional trans-Atlantic partners.”