British voters will cast ballots Thursday in elections that could hasten the end of Prime Minister Keir Starmer’s troubled term and confirm that an increasingly fractured United Kingdom has entered an era of messy multiparty politics.

Starmer’s center-left Labour Party is expected to take a battering in elections for local authorities across England and for semiautonomous legislatures in Scotland and Wales.

With the prime minister’s popularity in the doldrums from a weak economy and repeated questions about his judgment, rival parties are framing Thursday’s midterm votes as a referendum on Starmer and his 2-year-old government.…