The candidate for Reform U.K., the right-wing party of British lawmaker and ally of President Donald Trump, Nigel Farage, won a parliamentary by-election by six votes, taking the seat from the governing Labour Party in a stunning political upset.

Farage’s Reform UK is soaring in the polls—and doing better at the ballot box—as voters lose patience with slow economic growth and high immigration.

The upstart Reform UK party has emerged as a serious contender in British politics. The anti-immigrant populist movement has overtaken the main Conservative opposition to jostle with the ruling Labour Party for the lead in opinion polls,