CLACTON-ON-SEA, England — The campaign bus draws up and out steps a familiar figure in a smart suit and tie, who strides down the street, stopping at a pub, where he poses for pictures grinning with his pint, as he has done countless times before.

Nigel Farage — Britain’s most famous and pugilistic populist — is back on the trail.

Mr. Farage spent two decades promoting withdrawal from the European Union. When Britons voted for it in a 2016 referendum, and Prime Minister Theresa May and her Conservatives promised to see it through, he shifted his focus to media work, hosting a radio show and appearing on television news programs.

But the shambolic failure of attempts to deliver Brexit has given Mr. Farage another opening, and his newly founded Brexit Party threatens to become a guided missile aimed at Britain’s two main parties. Both are badly split over the question of Europe and both are already facing a backlash from voters.

Mr. Farage’s target is the elections to the European Parliament, normally a low-key contest in Britain. This time, it was not supposed to happen in the country at all, because Brexit was scheduled for March 29.