Mr. Johnson hammered again and again at the need to “get Brexit done,” casting it as the first step to solving everything from climate change to an erosion of trust in British democracy. Mr. Corbyn, on the other hand, refused to say even whether he wanted Britain to quit the European Union — arguing instead that the least disruptive way forward was returning the choice to Britons in a second referendum.

In a campaign that has seemed stuck in neutral, with Mr. Johnson’s Conservative Party holding a steady lead and Britons irritated by both main candidates, some analysts said that simply holding his own gave Mr. Corbyn a chance to turn the tide. But with Labour lawmakers antsy that their party is running out of time to close a double-digit polling deficit, it was not clear whether it would be enough.

Mr. Johnson is banking on Britons being so weary of a logjam in Parliament over Brexit that they overlook Mr. Corbyn’s criticisms and give Mr. Johnson the majority to push through his exit agreement. That deal made only halting progress in Parliament last month.

“Whether you voted for Leave or Remain, people want to get Brexit done, and to unleash the potential of this entire country,” Mr. Johnson said. “We have a deal that is oven-ready.”

Though the audience grew audibly annoyed with Mr. Johnson’s recurrent pleas to “get Brexit done,” the message makes up a big part of his appeal in a country that has watched lawmakers argue over the issue for three years, to little effect.