LONDON — Nigel Farage, who has fared better in fighting for Brexit than in winning elections, has said he will not run for a seat in Parliament in Britain’s general election in December. But he vowed to lead his Brexit Party in a vigorous campaign that could threaten Prime Minister Boris Johnson.

Mr. Farage said in a BBC interview broadcast on Sunday that he had concluded that he could advance the country’s exit from the European Union better by campaigning on behalf of his populist party’s other candidates than by contesting a seat himself.

“I don’t want to be in politics for the rest of my life,” Mr. Farage said, adding that he had thought “very hard about this.”

By giving up his own ambitions for Parliament, Mr. Farage, who has made much of his friendship with President Trump, could prove to be a bigger disruptive force in the election — and a bigger thorn in Mr. Johnson’s side — than if he were running himself.