President Obama officially endorsed Hillary Clinton in her bid for the White House with a social media blitz on Thursday, in keeping with a recent tradition of presidential endorsements coming until after the end of the primary process.

There’s a political calculation behind this decision that historians say goes back decades.

That’s because the question of endorsements was not relevant until late in the 20th century, said Barbara Perry, director of presidential studies at the University of Virginia’s Miller Center. This is partly due to the creation of primaries and caucuses, which gave voters more say on the nominee.

Presidents hold off on endorsing a candidate in an effort to stay above the day-to-day politics of the primaries.

“We view presidents as head of their parties,” Perry said. “That may not mean very much anymore, but it’s at the very least symbolic.”

Presidents may also wait now to avoid backing the wrong horse in a contested primary, in order to stay in the good graces of the eventual nominee. But once they do endorse, they are more active campaigners when they have a personal investment in their successor.

“Presidents typically get involved when they think their legacy is on the line. Barack Obama has good reason to think the Affordable Care Act is at risk should Donald Trump be elected; you can bet he will be working hard for Hillary Clinton,” wrote historian H.W. Brands via email.

Sometimes a president may be so controversial that a candidate from their own party may not want an endorsement too early because it could potentially hurt instead of help them.

For example, Vice President Al Gore did not use President Bill Clinton in his campaign due to the Monica Lewinsky scandal. George W. Bush endorsed his 2000 rival John McCain begrudgingly eight years later. But by then, Bush was so toxic that he did not attend McCain’s nominating convention and appeared only by satellite.

But when an endorsement is seen as a positive, there’s a benefit to waiting, since it gives the President the chance to tie things up after a contentious primary.

“A president wants to play the role of unifier for his party,” said Larry Sabato, director of the UVA Center for Politics, pointing out there’s only a couple of months until the election. “It just made sense for Obama to try to bring people back together quickly. Every day matters.”

President Obama’s endorsement, which came two days after the Associated Press declared Clinton the presumptive nominee, should help begin that process, added John Green, director of the Bliss Institute of Applied Politics at the University of Akron.

“It sends a very positive message to the president’s supporters,” he said, but he noted there’s a downside that Clinton will also inherit Obama’s critics. “The flip side is also true.”

This article was originally published on Time.com.