LONDON, June 10 -- A prime minister who commands his party does not need to threaten it. That is the cruel arithmetic hanging over Keir Starmer's latest move, a plan to tell his remaining ministers that supporting Andy Burnham's leadership ambitions means resigning from the government, reported by the Financial Times on Tuesday. The ultimatum is the act of a leader enforcing discipline. It is also the act of a leader who has run out of other tools.

The timing explains the urgency. On June 18, the voters of Makerfield, a Labour seat in Greater Manchester vacated by Josh Simons, will decide whether Burnham returns to the House of Commons, the one credential the Greater Manchester mayor needs to contest the leadership. Burnham stopped prete...