The results will determine how — and maybe even whether — Britain will exit the European Union, three and a half years after it voted to do so. The two largest parties running are Conservative and Labour.

Boris Johnson, the Conservative leader, is offering a scorched-earth version of Brexit, in which he aims to bulldoze political gridlock with the threat of a no-deal exit from the E.U. — something economists believe could disrupt the global economy — while avoiding details about the next phases of the likely yearslong divorce and trade-deal process.

In the final rounds of the campaign, he faced intense criticism over the future of the National Health Service, Britain’s cherished health system, which has not fared well under his party.

Jeremy Corbyn, on the other hand, would be Britain’s most left-wing prime minister in living memory. He promises to end austerity and wants a second referendum on Brexit (though he refuses to say which way he would vote).

Jewish leaders in Britain have accused him of tolerating anti-Semitism in the party, which he has denied and sometimes refused to address.