In choosing Kamala Harris as his running mate on Tuesday, Joe Biden checked the essential boxes his party had demanded–a woman, a minority, and a progressive who has moved left as the Democratic Party has. We’ll see how the California Senator plays in the swing-state suburbs that Mr. Biden needs to defeat President Trump.

Mr. Biden’s choice is especially important because he would be the oldest President on Inauguration Day at age 78. The actuarial tables and his declining mental acuity suggest he wouldn’t run for re-election, assuming he lasts a full term. Americans who have watched Mr. Biden on the campaign trail–and the way his advisers protect him from media questioning–are smart enough to know that in voting for Mr. Biden they’re also voting for his running mate as a likely President.

Ms. Harris is most appealing as an example of American upward mobility, especially for immigrants. Her father is a Jamaican-born Stanford economist. Her Indian-born mother was a breast cancer researcher at the University of California, Berkeley.

Even when the country was less racially tolerant than it is now, both parents had successful careers and were able to provide opportunities for their daughter even as they divorced. She made the most of them. Like Barack Obama, Ms. Harris’s success is a living refutation of the left’s critique of America as an oppressive, racist land.

Her political record, on the other hand, will reassure Democrats more than independents or soft Republicans. She’s a political lifer who rose through the patronage machine of former Assembly Speaker Willie Brown. She was a local prosecutor, a state Attorney General for six years, and was elected to the Senate in 2016 after party bigs cleared the primary for her. This isn’t an extensive resume for executive office, and on foreign policy she is about as experienced as Sarah Palin.

Ms. Harris ran for President this year but washed out quickly despite being a media favorite as the candidate from central casting. Her campaign’s most notable moment came in the first debate when she played the race card against Mr. Biden by distorting the history of forced busing in the 1970s. The jab scored oohs and aahs from the media judges but its demagoguery was blatant.

Mr. Biden will get credit in some quarters for rising above that attack to choose her. But we’d feel better about Mr. Biden if he had bypassed her for that reason. An effective President needs to put his stamp on the party, not vice versa. In choosing Ms. Harris, Mr. Biden is bending to the party’s preferences and rewarding the kind of political cheap shot he abhors in Mr. Trump.

Her record as prosecutor will bother some on the Black Lives Matter left, but her identity as a minority will blunt that concern. She’s progressive but malleable. She was quick to endorse Medicare for All and the Green New Deal as a presidential candidate, but she backtracked when they began to look too extreme.

She is also a ferocious partisan. As California AG she killed a deal that would have rescued some ailing Catholic hospitals because of opposition from the Service Employees International Union. In the Senate she was one of the nastiest questioners of Brett Kavanaugh, which is a high bar. She floated some innuendo about the judicial nominee’s alleged secret discussions about Robert Mueller’s Russia probe without any evidence. As a candidate, Ms. Harris will be delighted to brawl with Donald Trump.

Mr. Biden may have backed himself into the corner of having to choose Ms. Harris. He limited his choices by promising to select a woman, and the black Democrats who saved him in South Carolina pressed for a black woman. Then the Sanders wing pressed for a progressive, and Ms. Harris is a safer choice by far than Elizabeth Warren.

In this sense the choice is revealing about the unusual nature of Mr. Biden’s candidacy. He won the nomination as the last-ditch, anti-Trump alternative to what would have been the suicidal selection of Bernie Sanders. More than any recent nominee, Mr. Biden is a party figurehead, more than a party leader. In adding Ms. Harris to the ticket, he has underscored that a vote for Mr. Biden isn’t merely a vote to oust Mr. Trump. It’s a vote for the coastal progressives who now dominate the Democratic Party.

— The Wall Street Journal