Sound familiar? How 2016 Democratic race compares to 2008

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Bernie Sanders insists he will stay in the presidential race, in spite of the odds against him. He wants to stick around long enough to have an impact on the party's platform.

The 2016 Democratic primary race is echoing 2008, when Clinton was the Democrat who wouldn't drop out. Many Clinton supporters at this point eight years ago also said they couldn't image getting behind her opponent, then Senator Barack Obama, and there were real concerns about party unity then, too.

But what happened next created a template Democrats could follow this time, even though there are some roadblocks that didn't exist in 2008, reports CBS News correspondent Nancy Cordes.

Throwback Thursday: Hillary Clinton on her hu... 02:44

Hillary Clinton is familiar with the long process of uniting a fractured party.

In 2008, her primary fight with Obama also got nasty, like this exchange during a Democratic debate, when Obama accused her of distorting his record:

"Your husband did," Obama responded.

"Well, I'm here, he's not," Clinton fired back.

"OK, well, I can't tell who I'm running against sometimes," Obama said.

By the middle of May, Clinton was just over 100 delegates behind Obama, and virtually tied with him in the popular vote. "I am in this race, I am staying in this race because I believe," she said in New York on May 14, 2008. "The game isn't over, the election isn't over, the buzzer hasn't sounded."

In some polls, half of Clinton's voters vowed not to support her upstart rival. "They were the women, largely, who said they would never vote for Barack Obama... It was never going to happen -- 'party unity, my behind' was their attitude," recalled John Heilemann, who chronicled the 2008 race in the book, "Game Change."

In June 2008, after contests in all 50 states, Clinton finally bowed out and immediately called for solidarity.

Bernie Sanders' Rhode Island win doesn't help... 01:19

"I ask all of you to join me in working hard for Barack Obama, as you have for me," she said.

But this year, the policy differences between Clinton and Sanders are bigger, which could make party unity more elusive.

"People in the Democratic establishment recognize that the party has moved to the left -- that Sanders represents that leftwards shift -- and really importantly, that Sanders, not Clinton, is the one who has connected in a powerful way with the future of the party, which is to say young Democrats," Heilemann said.

And while Clinton had incentive to be a loyal soldier in 2008, Sanders could have more to gain by holding out on the issues that motivated his long-shot bid in the first place.

In an interview with Cordes, Sanders expressed that it was not enough that Clinton shared his concerns with some issues.

"We are going to go to the convention here in Philly and we're going to fight for a platform that represents the needs of working families," Sanders said.

The Clinton campaign knows it cannot expect Sanders to even tepidly make the case for Clinton until after he has ceded the nomination. The question is, when that will happen? Will it be when she crosses the delegate threshold in mid-May, after the last primaries in June, or will he hold out until the convention at the end of July?