Nigel Farage’s insurgent anti-immigration party, Reform U.K., scored a significant, if narrow, victory Friday in a parliamentary special election in the northwest of England, serving 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 a razor-thin margin of 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 was so close that the vote had to be recounted, delaying the declaration of the result for several hours. But the victory was the first of what could be an impressive show of strength by Reform in mayoral and local council elections held Thursday across England.