FROM THE moment former vice president Joe Biden became the Democratic Party’s presumptive nominee for president, one qualification has loomed as most important for his running mate: that she or he be prepared to serve as president. Sen. Kamala D. Harris, the California Democrat whom Mr. Biden announced Tuesday as his selection, meets that test.

Of course, in theory, that should always be the primary consideration for a vice president. Too often, though, candidates have been more influenced by electoral college arithmetic. There’s good reason to think that Ms. Harris, a gifted politician, can help lift the ticket, but California is not a state where Mr. Biden needs help. At 78, though, he would be far and away the oldest person to be sworn in as president, and demographic reality dictated a choice of someone who could plausibly step in.

Ms. Harris is such a person. She has been elected statewide three times in the nation’s most populous state. As California attorney general, running what amounts to a parallel Justice Department, she earned executive experience and respect for her savvy and administrative skill. As senator, she gained Washington experience. And as presidential candidate last year and this, she faced the pressures of the campaign trail and the debate stage.

It is a plus for the nation that the qualified person whom Mr. Biden settled on, after a fairly lengthy process, is also a woman, as he had promised, and a woman of color, the daughter of a mother from India and a father from Jamaica. Identifying as an African American, she would be the first woman and the first Black woman to serve as president or vice president. It is about time.

When she entered the presidential race, many handicappers picked her as the likely winner. The campaign that followed was something of a disappointment. Like Sen. Amy Klobuchar (D-Minn.), who also had served as a prosecutor before joining the Senate, Ms. Harris found herself somewhat wrong-footed by the Democratic Party’s turn toward criminal justice reform. She wavered on how much to embrace her own law enforcement record, as she did on Medicare-for-all and a few other fraught issues.

None of that is all that unusual in a first-time presidential candidate, and she exited the presidential field with her integrity and reputation intact. She showed herself to be a tough debater, including when she challenged Mr. Biden; it’s to both of their credit that they are ready to move on from that. She also exited as a plausible ideological soul mate for Mr. Biden, who has worked hard to accommodate the left wing of his party without acceding to all of its wishes.

Running to replace a president who has celebrated incompetence and elevated incompetents, Mr. Biden needed to choose a running mate who respects public service and has served well. In Sen. Kamala Harris, he has found such a partner.

Read more: