Initially she refused, or so the story goes.
It had been a bitter Democratic primary battle. Jabs on the campaign trial had turned to haymakers before Clinton finally acknowledged reality in June, and endorsed her rival.
"I ask all of you to join me in working as hard for Barack Obama as you have for me," she told an emotional crowd of supporters that night in Washington.
It was a speech many of her voters weren't yet ready to hear. The wounds would be slow to close. And the hurt feelings ran both ways: Obama supporters said Clinton had held on too long in the primary fight, potentially damaging the party.
But Clinton and Obama kept talking. When the two finally shook on the deal in December of that year, Democrats across the country exhaled. The phrase "a team of rivals" cluttered the cable news chyrons and added to Obama's Lincoln-esque aura in those early "new car smell" days of the presidency, when even Republican lawmakers were asking the first African-American president for his autograph.
Half a decade later, when they sat down together, side by side, for a "60 Minutes" interview before Clinton's departure as secretary of state, the former rivals spoke of each other as close friends.
"Look, in politics and in democracy, sometimes you win elections, sometimes you lose elections," said Clinton. "And I worked very hard, but I lost. And then President Obama asked me to be secretary of state and I said 'yes.' And so this has been just an extraordinary opportunity to work with him as a partner and friend, to do our very best on behalf of this country we both love."
Now, Clinton is casting herself as the defender of the administration she served, or as she told a crowd in Detroit recently: "We cannot let Barack Obama's legacy fall into Donald Trump's hands."
She has also questioned Sanders' loyalty to both the party and Obama's biggest accomplishments, namely the Affordable Care Act.
So what's the possibility for similar unity between Clinton and Sanders down the road, now that she is in striking distance of the nomination, but he keeps winning primaries?
Peace has seemed possible: After her April wins in Maryland, Pennsylvania, Connecticut and Delaware, she said: "Whether you support Sen. Sanders or support me, there's much more that unites us than divides us."
Since then, after tough losses in Indiana and West Virginia she's tried to placate his supporters, saying he should carry on his campaign as long as he likes. She did, after all, in 2008.
The final primary races don't occur until June 7 and the calendar and map mean Clinton can't reach the needed threshold of delegates until then. Even with a string of wins, the math is cruel for Sanders.
Meantime, the tone has also taken a turn. Clinton surrogates like Barbara Boxer have taken to publicly worrying if Sanders supporters will disrupt the convention in Philadelphia in late July.
Clinton and Sanders could do a lot to smooth things over in the weeks between June and the convention.
But don't get your hopes up for a Clinton-Sanders ticket, or even a "Secretary" Sanders, according to political watchers.
What you'll likely see is a leftward sway of the party's platform at the convention.
"In order for Democrats to unite behind Clinton, it is crucial that elements of the Sanders platform be incorporated into the party platform," says former New Mexico Gov. Bill Richardson, who served as energy secretary in the Clinton administration and chairman of the 2004 Democratic National Convention.
"Democrats are going to have to rally in support of increasing the minimum wage and other efforts to confront the issue of income inequality that has triggered widespread anger in both Democratic and Republican primary voters. For this to be achieved, however, it is vital that Sanders get behind Hillary soon, tone down the rhetoric, and begin to focus on the more realistic way for him to make a lasting impression on this election because winning the nomination is not happening," he said.
Still, Sanders is nowhere near ready to give up the fight, vowing to continue on to Philadelphia and railing against the Democrats' primary process. And Clinton could have an uphill battle winning over Sanders' supporters there.
"He has a lot of leverage going into the convention, because she needs his voters and they are like Trump's voters, somewhat cultist," says Carl Bernstein, CNN political commentator and author of "A Woman in Charge: The Life of Hillary Rodham Clinton." "What she needs from him is more than a grudging endorsement, but rather a message to his supporters, particularly to young people and especially young women, that they need to vote for Hillary Clinton."
But will his millions of supporters be satisfied with just inclusion in the platform?
"It strikes me as less likely that she brings him into the administration, because there seems to be more distance between them then there was between Clinton and Obama," said Robert Reich, the former labor secretary for the Clinton administration, and a strong supporter of Sanders. "And the distance here is more about anti-establishment versus establishment, it's not just policy. It's a more fundamental approach to the way American politics is organized and run. But if Clinton wins and she's smart, she'll do everything she can to embrace the movement that Sanders has channeled."
Another possibility: Clinton could bring aboard politicians that share Sanders' progressive streak, without having to tap the senator from Vermont himself.
"I personally think Elizabeth Warren would make a wonderful vice president," said Reich.