Surely, they say, Sanders' early momentum will fade, and a more traditional candidate will eventually take him down.
That was also the narrative that surrounded Trump at this point four years ago. Except even then, it was too late. Trump was well on his way to the nomination.
For Sanders' more moderate opponents, there may be a lesson in the experience of one of the establishment-backed candidates trounced by Trump in 2016.
"People don't realize how quickly things are moving and how far along things already are," said Tim Miller, the spokesman for Jeb Bush's 2016 presidential campaign, who sees strong similarities in the conditions facing Democrats opposed to a Sanders nomination to what happened in the 2016 Republican primary.
Miller added that he's seeing establishment Democrats repeat the same rationalizations that he and other Republicans made during the 2016 cycle.
"The idea that [Sanders] has a ceiling, the idea that there are two lanes of primary voters," Miller told CNN. "It doesn't work like that. Voters don't fall into neat lanes."
How Trump did it
What came next was a steady grind of wins and second-place finishes. Trump won South Carolina on February 20 with a similar share, 33%, and then blew out the competition three days later in the Nevada caucuses with 46% of the vote. Trump's next sets of primary wins showed his gradual but stable growth.
On Super Tuesday, Trump earned 34% of the Republican vote in 11 states, winning seven of them outright. Between March 5 and 12, he won 37% of the vote in 10 states and the District of Columbia.
By this point, the race was effectively over -- Trump was on his way to the nomination before his opponents knew what had happened. Sens. Marco Rubio and Ted Cruz and former Ohio Gov. John Kasich stayed in the race a while longer, but the trajectory was already clear.
Eight months later, Trump had won it all -- the plurality of the primary vote, the majority of delegates, the nomination of the Republican Party and even the presidency. There was no coalescence around a challenger to Trump, no contested convention, no significant protest to his nomination by GOP voters in November.
Instead, Trump did it with a steady stream of wins, collecting delegates, and slowly but surely consolidating support across the party. Trump would never earn a majority of the Republican primary vote -- he would end up with just under 45% -- but it didn't matter.
Plurality without majority
Like Trump, Sanders won't need to get a majority of votes in the primaries and caucuses to win the nomination either. Even if he continues to win primaries by small margins, or even come in second or third in a few of them, Sanders can wrack up delegates the way Trump did. The longer Sanders' center-left opponents remain in the race, the better it is for him.
And now that voting has started, there's the real possibility a snowball effect will redound to Sanders' benefit -- winning can beget more winning.
Despite Sanders' radical background, on top issues for Democratic voters like health care and climate change, he is well within the mainstream of the party. Sanders performs best with young voters, and campaigns with verve that may speak to Democrats' spirit of urgency in this moment.
Fear and loathing in Las Vegas and beyond
Still, establishment Democrats are focused on all the reasons Sanders can't or shouldn't win. Hillary Clinton, the 2016 nominee, gave voice to this view in a recent documentary series.
Democrats in Washington echoed Clinton's comments to CNN.
"His message in the party is of subtraction and division, not addition and multiplication," said one senior staffer for a moderate Midwestern House Democrat. "Their message is, 'If you're not with Bernie, you're stupid or wrong,' and that is not a message that builds a coalition in the presidential."
Another senior Democratic aide told CNN Sanders would be electorally disastrous against Trump.
"Bernie doesn't fit the bill. What Hillary said about him is absolutely true," said that aide.
Will anyone go after him?
While political professionals in Washington talk about the Sanders fantasy, just as they did with Trump, the alternative candidates are busy bashing each other rather than the front-runner.
If Democrats are to learn any lessons from the Republican experience in 2016, Miller says, they should be directing more attacks on Sanders himself. But there are also risks in going too hard against the front-runner -- backlash from his supporters who may make up more of the soul of the Democratic Party than establishmentarians had realized. There were concerns in 2016, for instance, that Clinton's protracted primary battle with Sanders depressed turnout from the Vermont senator's supporters in the general election.
"There is massive fear of crossing Bernie voters," Miller said. "It's a totally rational fear."
But there are signs the fear may be overcome, at least by some Democrats, by a fear of their own electoral prospects. Rep. Joe Cunningham, a first-term Democrat representing South Carolina's GOP-leaning coastal district, took a direct swipe at Sanders just hours after his victory in New Hampshire.
CNN's Jamie Gangel, Arlette Saenz and Jeff Simon contributed to this story.