The downside is that Mr. Starmer will have to battle for visibility at a time when the coronavirus crisis is eclipsing all other news and Mr. Johnson is commanding the headlines. The prime minister’s health has been a front-page story since he went into isolation last week with a fever after catching the virus. On Saturday, Mr. Johnson’s 32-year-old girlfriend, Carrie Symonds, who is pregnant, disclosed on Twitter that she, too, is suffering symptoms.

Even if Mr. Starmer manages to break through, the electoral math for Labour is daunting. Given the Conservative Party’s majority of 80 seats, it would take a political earthquake to bring Labour to power in the next elections, scheduled for 2024. For now, Mr. Johnson faces no realistic prospect of losing votes in the House of Commons.

Nor is history encouraging: The last opposition Labour leader to become prime minister was Tony Blair in 1997, and Mr. Starmer is not viewed as a political talent on Mr. Blair’s level (even if he is now reviled by many Labour members because of his role in the Iraq war).

Mr. Starmer cemented his reputation for being a bit dull when an interviewer recently asked him the most exciting thing he had ever done. “Oh, I’ve done lots of exciting things,” he replied. “Playing football, going to football with my kids.”

But having performed well as director of public prosecutions and shown he is fast on his feet in debates in the House of Commons, he has already passed one critical test that Mr. Corbyn, the departing leader, failed: looking like a potential prime minister.

Mr. Starmer’s competent, cautious political style equips him better than most for the task of fixing Labour’s bitter ideological divisions. Before the election, he proved his diplomatic mettle by nudging the party’s Brexit policy toward support for a second referendum, without ever falling out with Mr. Corbyn, who was never an enthusiast for that cause.