Hillary Clinton is still holding auditions for vice president, but her most important running mate for the balance of the campaign will be President Obama, for better or worse.

After Mr. Obama endorsed his former secretary of state last week, this newspaper reported that no sitting president in recent history “has campaigned for his party’s nominee as much as Barack Obama plans to for Hillary Clinton.” Mrs. Clinton wouldn’t have it any other way. Her campaign has been nothing if not an assiduous appeal to the young, minority and women voters who put Mr. Obama over the top in 2008 and 2012. And Mr. Obama knows that his legacy would be badly tarnished by a Clinton defeat in November.

While her Democratic primary opponent, Bernie Sanders, has drawn sharp contrasts with the administration on issues such as health care, Mrs. Clinton has by and large embraced Mr. Obama’s agenda in hopes of keeping his supporters happy. When she has broken with the president, as she did in opposing the Pacific trade accord last year, she has done so as delicately as possible. The only question for Team Clinton and the Democrats is whether Mr. Obama’s playbook will work for someone else.

By some measures, the president is well positioned to be an asset in the fall campaign. Unlike his predecessor, George W. Bush, who left office deeply unpopular and with the economy in a free fall, Mr. Obama’s favorability rating currently rests around 50% overall and 80% among registered Democrats. With those kinds of numbers, Mrs. Clinton isn’t the only Democrat who will want the president on the stump this fall.

Still, making common cause with Mr. Obama has a potential downside for Mrs. Clinton. He’s not on the ballot this year, and bad things have happened to the party on his watch. “Democrats in the Obama era,” the political scientist Larry Sabato wrote in 2014, “have racked up net forfeitures of 11 governorships, 13 Senate seats, 69 House seats, 913 state legislative seats, and 30 state legislative chambers.” They lost control of the House in 2010 and the Senate four years later.