Those clashing messages — the first from Prime Minister Boris Johnson, the second from the Labour Party leader, Jeremy Corbyn — formed the core of the first debate before next month’s general election, which could determine whether Britain leaves the European Union or stays.

Labour trails the Conservative Party in polls by an average of a dozen percentage points. But with Britons irritated by both main candidates, some analysts said that simply holding his own gave Mr. Corbyn a chance to turn the tide.

U.S. debate: Ten Democrats seeking the presidential nomination take the stage in Atlanta tonight. Four of them lead the pack: former Vice President Joe Biden, Senator Elizabeth Warren of Massachusetts, Mayor Pete Buttigieg of South Bend, Ind., and Senator Bernie Sanders of Vermont.