And if Mr. Johnson does not agree, he said Friday, the Brexit Party will field 500 candidates in the election, which could split the pro-Brexit vote and cost the Tories several close seats.

Mr. Farage could also hammer Mr. Johnson for failing to pull Britain out of the European Union on Oct. 31, as he promised, or for his deal with Brussels, which Mr. Farage claims will align Britain too closely with Europe and, as Mr. Trump said, impede a trade deal with the United States.

“It’s all about control of the narrative,” said Timothy Bale, a professor of politics at Queen Mary University of London. “If Johnson can keep the message on getting Brexit done, and if Corbyn can’t counter with his anti-austerity, populist message, the Conservatives will probably win.”

Mr. Johnson is starting out with an undisputed advantage. A poll released Thursday by Ipsos Mori showed the Conservatives leading Labour by 41 percent to 24 percent nationally. That would translate into a 108-seat Conservative majority in Parliament, according to Mr. Travers. These are the kind of numbers that emboldened Mr. Johnson to call an election now.

The Conservative Party is going after a swath of Labour seats in northern England and the Midlands, where people voted for Brexit in the 2016 referendum and are frustrated that it has not happened.

The party has identified a target voter, named “Workington Man,” for the coastal town of Workington, in northwest England, which is surrounded by the remnants of the coal and steel industries. He is older, white, not college-educated, a lover of the rugby league and a Labour voter who supported leaving the European Union and feels left behind by the Britain of today.

These voters care more about economic security than individual liberty. They favor putting additional police on the streets and toughening immigration policies, according to a study by the right-wing think tank Onward, which coined the phrase Workington Man.