As Clinton locked up the Democratic presidential nomination — by winning millions more votes, racking up hundreds more pledged delegates and garnering the backing of the vast majority of superdelegates — Sanders' supporters chanted "Bernie or Bust" and booed Clinton's image on the television screens broadcasting election night coverage at the senator's rally in Santa Monica, California.
It was a sign the party unification process may not come quickly or easily as Democrats gear up for a contentious general election against Donald Trump.
And it's why Democrats are likely to look to a familiar face to try and quell the simmering anger from the army of young people who are still "feelin' the Bern": Elizabeth Warren.
Aside from President Barack Obama and Vice President Joe Biden — whose positions in the White House and popularity among Democrats will make them powerful figures in the coming unification process — Democrats say Warren's star among Sanders' left-wing coalition will make her an important figure as the party attempts to come together ahead of the Democratic National Convention in July.
"Certainly Warren is one of the few national figures that has the ability to have a broad appeal across different factions of the party, and the more active she is I think certainly will give comfort to the Sanders people," Steve Schale, a Florida Democratic strategist who led President Barack Obama's 2012 re-elect in the Sunshine State, said in an interview.
Neutral party: Warren — the Massachusetts senator, scourge of big banks and progressive icon — was the leader of the Democratic Party's left-wing movement before Sanders' meteoric rise in the primary.
While she stayed neutral throughout the primary, despite the best lobbying efforts by both camps, many Democrats now expect her to come out and endorse Clinton's bid as the process comes to a close.
She's already indicated that she does not believe superdelegates should overturn the popular vote — which is Sanders' only hope at the nomination after Clinton clinched the majority of pledged delegates Tuesday night.
And they say she'll be a vital surrogate who can attack Trump on issues progressives hold dear — an ability Clinton now lacks after the acrimonious primary fight.
"She's going to play that middle person," one chief of staff to a Democratic member of Congress said in an interview. "It may not be going out there every day with a full throated endorsement of Hillary Clinton, but she'll go after Trump on principled issues that progressives care about."
The biggest surrogate role of all? Warren's potential unifying qualities have also stoked speculation that she could not only be a top surrogate for Clinton campaign, but that she could also grace the ticket as Clinton's running mate.
In recent weeks, Warren has come out in virulent opposition to Trump, both in interviews and stinging Twitter rants that got under Trump's skin.
Her willingness to be an attack dog for her party caught the eye of both the Clinton campaign and other Democratic officials such as Senate Minority Leader Harry Reid — who reportedly already looked up the rules to fill Warren's Senate if she's elected vice president in November.
With the primary over and as Clinton's campaign assesses the extent of the damage the primary inflicted on her campaign, Democrats say there's a growing possibility she could be Clinton's running mate.
"If Clinton chooses Warren as VP, I think the Bernie people would shut up over night," said the House chief of staff. "I think it would be a game changer for progressives, but the calculous is what they're seeing they need to win."