The daunting delegate math. His loss to Hillary Clinton in Tuesday's crucial California primary. And now, U.S. President Barack Obama's endorsement of his rival.

Bernie Sanders has plenty of reasons to quit the race for the Democratic presidential nomination — and do it quickly. But with five months to go until the general election, the party also has reason to let Sanders withdraw at his own pace: Namely, to help Clinton's bid for the White House.

It's all a matter of time for Sanders, campaign analysts say. But that's time for Sanders to exercise what political leverage he has left to influence the party platform. Time to lead his supporters towards acclimating to Clinton. And, as Vice-President Joe Biden put it, time to "decide on his own" how to exit gracefully.

If that's what Sanders needs, the Democratic Party will want to accommodate, says Joe Trippi, who ran Howard Dean's campaign for the Democratic presidential nomination in 2004. Too much pressure too soon, and the party could mishandle its opportunity to retain the current Bernie-or-bust crowd.

"You can't expect someone who's been running 100 miles an hour in one direction to stop full stop, and turn around to run 100 miles an hour in the opposite direction in support of Hillary Clinton," Trippi says. "If Sanders did it that way, it would make it harder to lead the Sanders supporters to make that turn."

Those millions of progressive-leaning Sanders supporters are the ultimate spoils of this marathon primary season. Clinton can afford a little patience, Trippi says, noting that she held onto her shot at the 2008 nomination against Obama until the last primaries.

By the time of the Democratic National Convention that year, Clinton was backing Obama and Democratic unity.

Trippi is comfortable waiting if it means a better shot at preventing presumptive Republican nominee Donald Trump from taking the White House.

Democratic presidential candidate Bernie Sanders walks with U.S. President Barack Obama to the Oval Office. The Democratic hopeful faces mounting pressure to suspend his campaign as it becomes apparent Hillary Clinton will be the party's nominee. (Gary Cameron/Reuters)

"Sanders wants to stay true to who he is, and to help defeat Trump. Give him room to decide how best to help stop Trump," Trippi says.

In his experience with the Dean campaign, hard-fought primary battles don't just end abruptly.

"If there's just a pin of light still at the tunnel, it's amazing how bright that light seems for the candidate."

No 'thumbs on the scale'

Obama speechwriter Jon Favreau thinks Sanders has signalled he knows what lies ahead.

"If Bernie truly intended on contesting the convention, he would've kept attacking Hillary. He didn't," Favreau tweeted on Tuesday, following the Vermont senator's disappointing primary night in California. "[He] just needs time to land the plane."

If Bernie truly intended on contesting the convention, he would've kept attacking Hillary. He didn't. Just needs time to land the plane. —@jonfavs

For a moment yesterday, it seemed like Sanders might be beginning his final approach. He struck a somewhat conciliatory tone at the White House, telling reporters shortly after his meeting with Obama that he looked forward to meeting with Clinton "to see how we can work together and defeat Trump."

The president and vice-president have maintained a stoic outlook on the nomination race, and Sanders thanked them for it.

"What they said at the beginning is that they would not put their thumbs on the scale, and in fact, they kept their word," he said. "I appreciate that very, very much."

U.S. President Barack Obama endorsed Hillary Clinton for the Democratic presidential nomination on Thursday, after speaking with her rival Bernie Sanders. (Gary Cameron/Reuters)

His reassurance to supporters that he will still compete in Tuesday's Washington, D.C., primary is one way to delay the inevitable. It also allows Sanders to keep touting his signature issues such as greed on Wall Street, crumbling infrastructure, quality of life for Native Americans and spiralling college debt loads.

Joshua Grossman, president of the pro-Sanders SuperPAC Progressive Kick, expects Sanders to command a prime slot at next month's Democratic National Convention in Philadelphia to express his desire for economic transformation.

Joe Trippi, left, consults with former Vermont governor Howard Dean in this 2003 file photo. Trippi says fading campaigns such as that of Bernie Sanders experience a natural slowdown before candidates can make a graceful exit. (Jason Reed/Reuters)

Although Sanders vowed to bring his message to the convention, he didn't take reporters' questions yesterday, leaving it unclear about whether he'll still be an active candidate at that point.

One way or another, Grossman feels Sanders deserves to wield some influence with the party's platform-drafting committee. That could go a long way to satisfy Sanders supporters.

"Especially for the passionate young people who are new to politics and voted for Sanders, letting them know their voices are heard," he says. "It all falls under treating someone with respect."

Ryan's 'hostage video' endorsement

As for where things go from here, it has to be about party unity, says Jeanne Zaino, who teaches campaign management at New York University.

So far, even with Sanders still in the race, she says there's a "marked contrast" between how the Democrats and Republicans have managed to coalesce behind their presumptive nominees.

"Obama's endorsement for Clinton came in the form of a very supportive, upbeat video" filmed in the Oval Office, Zaino says. But Republican House Speaker Paul Ryan's video for Trump "reminded some people of a hostage video."

Obama's video message also underscored the unity theme, highlighting his once-fraught relationship with Clinton.

Democratic presidential candidate Hillary Clinton appears to have a lock on the nomination. (Drew Angerer/Getty Images)

"I don't think there's ever been someone so qualified to hold this office," the president said, noting he also debated her more than 20 times during a bitter 2008 nomination battle.

Ryan, the Republican Party's third most important elected official, this week slammed Trump's remarks about a judge of Mexican heritage as "racist."

"These kind of antics are distracting," Ryan said of Trump's comments.

Although Obama filmed his endorsement on Tuesday, it was only published after his meeting with Sanders on Thursday.

That rollout, as well as Sanders's three conversations this week with Obama and reports he will soon cut half his staff, suggest his campaign is winding down gradually.

It's now up to him to figure out an ending that will work for his legacy as well as the Democratic Party, Zaino says.