The turmoil inside the Republican Party under Donald Trump's divisive candidacy is being mirrored across the Atlantic in Britain, where another outsider is confounding political experts.
The U.K.'s opposition Labour Party on Saturday reelected socialist Jeremy Corbyn as leader, despite warnings from horrified party grandees such as ex-Prime Minister Tony Blair that he is incompetent and will never win a nationwide poll.
Centrists fear that Corbyn's emphatic victory, after a bruising and bitter campaign, will keep Labour out of power for a generation and could even trigger a permanent split in the 116-year-old party.
Labour's woes echo the strife at the other end of the political spectrum across the pond, where several senior GOP figures have refused to endorse their own party's presidential candidate and fault lines have emerged between lawmakers, donors and grassroots supporters.
Like Trump, Corbyn has broken unwritten elections rules, defying observers who assumed party members would settle on mainstream, voter-friendly establishment candidates.
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The 67-year-old socialist and peace campaigner has spent his entire career on the sidelines, voting against policies that polls suggest voters would support. He is lukewarm on the U.S. special relationship and is opposed to nuclear weapons — including the costly, U.S.-maintained Trident nuclear missile system.
Corbyn also is a critic of NATO and Israel. In 2009, as patron of the Palestine Solidarity Campaign, he invited members of Hezbollah and Hamas to the House of Commons where he called them "friends" — a term he recently said he regretted using.
And like Trump, Corbyn is wildly popular with his core supporters despite the detractors — more than doubling Labour's membership registrations over the past year and pulling huge crowds to political rallies.
But his opinion poll ratings among wider voters are historically low. Even in the mid-term of an election cycle, Labour is trailing the ruling Conservatives by an average 10 percentage points.
This has left Britain's opposition engaged in a bitter and vitriolic internal feud that has seen accusations of anti-Semitism and even death threats, giving their rival Conservatives free rein to pursue austerity policies and push through the controversial Brexit from the European Union.
John McTernan, a former Blair adviser and a persistent Corbyn critic, on Wednesday called him "the most woeful, the most useless, the most dangerous and the most unpleasant leader the Labour Party has ever had in its history."
Party members disagreed this weekend, giving him 61.8 percent of the vote.
Saturday's vote — at the party's conference, which is akin to the conventions in American politics — came after exasperated centrist Labour lawmakers attempted to give Corbyn the boot. In June, 172 Labour members of parliament — almost two-thirds of the entire elected party — declared they had no confidence in the leader.
Yet his supporters — mostly trade unions and new, ordinary party members — say centrists have misread the public mood. They contend that Corbyn's low-key style and focus on inequality and social justice — far to the left of Bernie Sanders — represent a refreshing break from establishment politics.
"We have been ... involving people in politics that matters," Corbyn told his supporters at a final rally.
Even with the leadership question settled, the future of the party is far from certain.
"This situation is very, very unusual in British politics," said Richard Whitman, a professor of politics at the University of Kent in England and a senior fellow at the Chatham House think tank. "Unlike in American politics, the thing you normally need as leader is to retain the confidence of your parliamentary party. When [former Prime Minister] Margaret Thatcher lost the confidence of her members of parliament, she had to go.
"Corbyn is in a position in which the normal rules do not seem to apply and where he'll be leading a parliamentary party that doesn't believe in him but where he has support from base activists."