Nigel Farage’s insurgent anti-immigration party, Reform U.K., scored a significant, if razor-thin, victory Friday in a parliamentary special election in the northwest of England. The result served notice that Mr. Farage, a populist fixture and close ally of President Trump, is again a rising force in British politics.

Reform’s candidate, Sarah Pochin, won by just six votes over her Labour Party opponent, Karen Shore, in Runcorn and Helsby, seizing what had been a safe seat for Labour until the incumbent, Mike Amesbury, was forced to resign after being convicted of assault for punching one of his constituents.

On a night of high drama, the outcome — the tightest in such an election in modern history — was so close that the vote had to be recounted, delaying the declaration of the result for hours.

But the victory, by 12,645 votes to 12,639, was the start of what could be an impressive show of strength by Reform in mayoral and local council elections held Thursday across England.

More than 1,600 municipal seats are up for grabs, and polls suggest that Reform could win at least 300 of them.

If Reform’s gains are borne out as the ballots are counted throughout Friday, it would deliver a jolt to British politics, potentially accelerating the country’s shift toward a more polarized, multiparty system.