On June 7, 2008, Hillary Clinton made way for Barack Obama to conquer the White House, conceding defeat after a fractious primary race, and throwing her full weight behind him.Eight years later, it is she who faces the arduous task of unifying her camp after a -- sometimes bitter -- battle with Bernie Sanders.On paper, it seems an easier task than the one facing her Republican rival Donald Trump whose insurgent candidacy and incendiary comments -- dubbed "racist" by leading GOP figures -- are tearing his party apart.But Clinton's challenge remains very real, with one question still unanswered: how will "Bernie" behave towards the nominee he fought tooth-and-nail to stop?Will the Vermont senator bow out with grace? Will he drag out the battle all the way to the July Democratic convention in Philadelphia -- to drive home his message about reforming Wall Street, the minimum wage and the role of money in politics?The silver-haired 74-year-old has repeatedly sworn to fight on until the end.After decades of relative isolation in Congress, this campaign has handed him, and his calls for a political revolution, a belated -- but spectacular -- dose of recognition."Today's primaries are no longer about the nomination. They are about leverage in the peace talks that are sure to follow," the one-time Obama strategist David Axelrod tweeted on Tuesday.Delivering her victory speech in New York, Clinton did not directly urge her rival to stand down. But she held out an olive branch to him -- and his supporters."Let there be no mistake, Senator Sanders, his campaign, and the vigorous debate that we've had about how to raise income, reduce inequality, increase upward mobility, have been very good for the democratic party and for America," she said."As we look ahead, let's remember all that unites us."In 2008, Clinton had thrown her weight unequivocally behind her erstwhile rival, calling for her supporters "to take our energy, our passion, our strength, and do all we can to help elect Barack Obama, the next president of the United States."This time, Clinton will have to earn the support on November 8 of the young, passionate voters fired up by Sanders' grassroots campaign.Her biggest hurdle: the sizeable share of Bernie's fans who view the former first lady, senator and secretary of state as calculating and insincere -- echoing the favorite line of attack of her Republican rival Trump."Clinton doesn't have the ability to do it alone," argued Larry Sabato, a political analyst from the University of Virginia. "A sizable minority of the Sanders troops have grown to despise her. Sanders will have to back Clinton repeatedly and with enthusiasm."Democrats like to point out that the bad blood between the Obama and Clinton campaign teams was considerably worse than between this year's rival camps -- and that it did not stop them from winning.They are also counting on Obama himself -- who is expected to enter the race in the role of unifier-in-chief as soon as a winner emerges from the primaries."I think you can expect the president to play that kind of role," White House spokesman Josh Earnest said recently.Obama's message has already been fine-tuned: the two Democratic candidates, while very different in style -- are not so different when it comes to substance."They're both good people. I know them both well," Obama said during a trip to Japan last week -- as if laying the groundwork for the necessary reconciliation ahead. "I think that it's important for us to try to end this in a way that leaves both sides feeling proud of what they've done."For Sabato, "President Obama must reason with his party's left wing, using fear of Trump as the emotional appeal."According to the latest Gallup poll, Obama's popularity with that segment of the electorate is sky-high -- at 92 percent.Eight years apart, Obama and Clinton have one asset in common: both offer voters the chance to write a new page in American political history, as the first African American in the White House, and the first woman US president.Whatever the weaknesses of a candidate whose stiff campaign style has struggled to ignite crowds, Democratic voters could yet mobilize massively to prove that their party is, once again, the one that pushes the boundaries in American society.