Democrats are worried that if he doesn't win the party's presidential nomination himself, Sen. Bernie Sanders (I-Vt.) will be a thorn in the side of the Democratic nominee for president - especially if it's former Vice President Joe Biden.

The Democrats worry that Sanders won't be a team player if he loses, and that he and his supporters will be particularly problematic if Biden or another rival seen as out of step with progressives comes out on top.

"I think Bernie will do everything in his power to elevate himself by pushing others down," said one aide to a Democratic senator.

A senior aide to a second Democratic senator said concerns about Sanders dividing the party next year are widely held, even though everyone in the Senate Democratic Conference is holding their tongue for fear of making things even worse.

The aide said that when it comes to Sanders, there is a concern among Biden supporters that "this guy is going to play spoiler again."

The worries follow a bitter 2016 primary fight between Sanders and former Secretary of State Hillary Clinton, who won the nomination and then was defeated by President Trump in the Electoral College.

Some Democrats have blamed Sanders in part for that outcome, arguing he stayed in the primary against Clinton too long and that he did not do enough to bring his supporters on board in November.

For their part, Sanders supporters are suspicious of a Democratic establishment they think rigged the game against their candidate in 2016.

They also say those wringing their hands over Sanders are being disrespectful by assuming he won't win. They note he isn't far behind Biden in national polls or those in Iowa and New Hampshire.

Rep. Ro Khanna (D-Calif.), a national co-chair of Sanders's campaign, said he "has a very good chance of being the nominee."

"His support is remarkably stable and he continues to attract huge crowds. He has a very committed base. I anticipate he's going to do very well in Iowa and New Hampshire," he said. "If he's not the nominee, just like anyone who's not the nominee, I guarantee you he and people around him will go all-in to support whoever the nominee is."

Ben Tulchin, Sanders's campaign pollster, accused Democrats of underestimating Sanders, noting that Biden's poll numbers drifted downward from December to mid-April when he announced his candidacy before he got a bump.

"He got a bounce from getting in," he said. "But there's no reason, if those voters left him once before, they won't leave him again."

In any event, Tulchin said that if Sanders does fall short, he's committed to beating Trump.

"A driving force for a lot of Democratic primary voters is the desire to beat Trump. Bernie shares that desire and says his No. 1 goal is defeating Trump. He obviously hopes to be the nominee but he'll get behind any person who is out there. I think we'll have a very unified party," he said.

Sanders has loyal supporters and a proven fundraising track record, leading many of his colleagues to believe he will be in the race for the long haul.

"The question is how much resiliency you have," said one Democratic senator, who then questioned whether candidates such as former Rep. Beto O'Rourke (D-Texas) and South Bend, Ind., Mayor Pete Buttigieg (D) will retain support if they falter in polls.

"Can you keep on going back to people for donations?" the Democratic senator asked. "That's what I think Bernie has. I don't know for the others like Buttigieg and Beto."

Sanders has seen a dip in support, according to polls.

A Morning Consult poll from earlier this month found that Sanders's support among people ages 18 to 29 dropped from 45 percent in March to 33 percent in May.

And a recent Monmouth University poll showed Sanders's support among Democratic-leaning voters dropping from 25 percent in March to 20 percent in April to 15 percent in its latest survey.

The loyal base of supporters, however, could prevent Sanders from falling too far, say Democratic lawmakers. This could be particularly helpful in the large Democratic field, which includes two dozen candidates.

There are also questions among Democrats about whether Biden can retain his pole position.

"Muskie was an unbelievable front-runner," said the Democratic senator, referring to Sen. Edmund Muskie (D-Maine) and his 1968 run for the party's nomination.

"He had the stature and was younger than Biden and just cratered," the lawmaker warned.

A second Democratic senator said that while Biden has had a good campaign rollout and maintained strong poll numbers, there are doubts about whether he has staying power.

"I am not persuaded that his polling lead is not written in the wind," the lawmaker said.

If Biden collapsed, it would be a boon to Sanders, who has consistently placed second in recent Democratic primary polls.

Jim Kessler, a former aide to Senate Minority Leader Charles Schumer (N.Y.) and executive vice president for policy at Third Way, a centrist Democratic think tank, said he has heard concerns about Sanders becoming a thorny problem for the party's eventual nominee.

"I expect him to stay in the race all the way until the end, for a couple of reasons. One is he did it last time, and he stayed in well beyond the point where there was any doubt about the outcome," he said. "He'll raise enough money to do so. He's never been loyal to the Democratic Party and it remains to be seen how helpful he would be to the Democratic nominee if it's not him in 2020."

"He wasn't real helpful to Hillary," Kessler said.

"Should his candidacy founder, the hope is that when it comes time, he bows out gracefully. The stakes are enormous," he added.

Charles Chamberlain, the executive director of Democracy for America, which endorsed Sanders ahead of the 2016 Democratic primary, said Biden is already showing signs of weakness.

"If you actually see the campaigns on the trail, there are a lot of campaigns that are getting a lot more people than Joe Biden is. Joe Biden's campaign rallies are eerily reminiscent of Hillary Clinton campaign rallies where there are small numbers of people compared to massive rallies of thousands of people for everybody from Bernie to Harris to Warren," Chamberlain said of Sanders and Sens. Kamala Harris (D-Calif.) and Elizabeth Warren (D-Mass.).

"The reality is we've got a long campaign ahead of us with a lot of time and if I look at the support for Bernie versus Biden on the ground, from campaign rallies to the supporters that we see that are active and involved, that Bernie base ... the Bernie base is strong," he added. "They're not going anywhere. That base is big, it is wide and they are committed to their candidate."

"I'm not seeing that for Biden. There's some hardcore establishment big-money fundraisers that are really pushing for Biden hard," Chamberlain said, arguing that Biden has a "shallow level of support" from many Democrats who see him as electable.