What’s the likely impact on British politics?

Earlier this year, when Mr. Sunak’s leadership looked shaky, these elections seemed like a potential trigger for a leadership crisis and a comeback opportunity for Mr. Johnson, whose own fall was accelerated by local election losses last year.

Since then, Mr. Sunak has struck a post-Brexit deal with the European Union on Northern Ireland, and stabilized the economy after upheavals under Liz Truss, Mr. Johnson’s short-lived successor. By contrast, Mr. Johnson is embroiled in an inquiry into whether he lied to Parliament about lockdown-busting parties during the pandemic.

So Mr. Sunak’s position looks secure for now. But a bad result could demoralize party workers, shake confidence in his prospects, embolden his critics and confirm expectations that he will postpone calling a general election until late next year (it must take place by January 2025). A better-than-expected result for the Conservatives would strengthen Mr. Sunak and increase pressure on Mr. Starmer.

If the Conservatives do suffer, the prime minister has one big thing going for him: timing. On Saturday, all the British media’s attention will shift to the pomp and pageantry of the coronation of King Charles III.