Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

If it weren't for Donald Trump, Bernie Sanders would be the talk of the 2016 presidential race right now.

The Vermont senator is surging in the polls, he's getting some of the biggest crowds and he's increasingly become an obstacle to Hillary Clinton's presidential ambitions.

But there is something notable Sanders is missing – endorsements from the colleagues who know him the best.

Strikingly, Sanders has to yet win an endorsement from a sitting Democratic senator, House member or governor, according to FiveThirtyEight.com's endorsement tracker.

By contrast, Clinton has racked up endorsements from 30 senators, seven governors and more than 100 House members – including the top politicians from Sanders' home state of Vermont: Sen. Pat Leahy, D-Vt., and Gov. Peter Shumlin. (Vermont's other major statewide politician, Rep. Peter Welch, D-Vt., has yet to endorse.)

To be sure, much of Sanders' appeal is that he's a political outsider. Indeed, he's technically an independent who caucuses with Senate Democrats, and he's running for the Democratic nomination because he sees it as the best path to winning the White House.

“The American people, in my strong view, are sick and tired of establishment politics, of establishment economics. And they want a candidate who is prepared to stand up to the big money interests, Wall Street, corporate America, that exert so much power over our legislative life in Washington,” Sanders said on “Meet the Press” last Sunday.

But like Trump, Sanders faces this question: Can he succeed winning a party's presidential nomination when the politicians who lead that party aren't supporting him?

Why Democratic politicians are backing Clinton over Sanders

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Back in June, Sen. Claire McCaskill, D-Mo. – who has endorsed Clinton – claimed on MSNBC’s “Morning Joe” that Sanders is too liberal to win the White House. “I think Bernie is too liberal to gather enough votes in this country to become president,” said McCaskill, who was first elected to the U.S. Senate the same year Sanders was back in 2006.

Another member of the Democratic Class of 2006, Sen. Sheldon Whitehouse, D-R.I., says he’s backing Clinton over Sanders because she is the “best choice.”

“I love Bernie, and I love the way he is out there shining a giant spotlight on the big interests trying to seize even more for themselves by controlling our politics,” Whitehouse said in a statement to NBC News. “But at the end of the day, I think Hillary is simply the best choice for the presidency."

Leahy, Sanders’ fellow Vermont senator, also has endorsed Clinton, promising it to her before Sanders ever got into the race. “He gave Hillary Rodham Clinton his word long ago, before Sen. Sanders indicated he was going to run,” a Leahy spokesman tells NBC.

And Shumlin, Vermont’s governor, is backing Clinton because he believes she’s the best person for the job. “The governor has tremendous respect for Sen. Sanders, and doesn’t have a bad word to say about him,” said Scott Coriell, Shumlin’s communications director.

“But he think s Hillary Clinton is the right person to lead American going forward,” Coriell adds.

Why endorsements matter

According to political scientists, endorsements at this stage of a presidential race are more predictive than the early polls about who wins a party’s nomination.

"In the 1980-2004 primaries, a candidate's share of endorsements during the invisible primary was associated with how many delegates that candidate won in the party convention months later," political scientists John Sides and Lynn Vavreck write in their book, The Gamble. "Why endorsements matter in this way is still unclear, but they are certainly a prominent and important signal about candidates' standing with the party and ultimately whether a candidate can be the party's standard-bearer."

And what does that say about Sanders? Or even Trump?

“It’s very early, the landscape is still shifting, but that no one jumps on board right away — even someone from a home state — is not a good sign of people’s confidence in a person’s abilities to either do the job or to win the nomination/election,” Vavreck, who teaches at the University of California (Los Angeles), emails NBC.

Sanders does have endorsements from these two men -- Ben & Jerry

That said, Sanders picked up an endorsement from Larry Cohen, former president of the Communication Workers of America.

But the campaign acknowledges that Sanders doesn’t have an endorsement from his colleagues in Congress or in Vermont.

“We have not announced any endorsements from members of Congress or top elected officials in the Vermont,” said Sanders spokesman Michael Briggs. “We do have Ben and Jerry” -- referring to the founders of the famous ice-cream company.