2020 Democratic presidential candidate and U.S. Senator Bernie Sanders (I-VT), speaks at the 2019 National Action Network National Convention in New York, April 5, 2019.

Sen. Bernie Sanders is making mainstream Democrats nervous.

But some of the presidential candidate's supporters say it's not because they're worried Sanders can't defeat President Donald Trump in 2020. It's because he can, they say.

The Vermont democratic socialist may have surprised the party's establishment with his strong performance so far. He's been polling second, behind only former Vice President Joe Biden, who has yet to enter the race. Sanders also took the lead in an Emerson poll released Tuesday. He raised more than $18 million in the first quarter of 2019, the most money of any Democratic contender. And his base of loyal followers, who made him a force in the 2016 primary, has not gone anywhere.

"Bernie right now has a better chance of winning than the rest of field combined," said Cenk Uygur, founder of progressive news network TYT and host of "The Young Turks," who supported Sanders in 2016. Uygur has not decided whom he will back in 2020, but says he likes Sanders and Sen. Elizabeth Warren, D-Mass.

Uygur said the establishment's concerns about Sanders raised in a New York Times article on Tuesday, will only make Sanders stronger.

The article quotes party insiders who fear that Sanders' campaign will complicate efforts to defeat Trump. Their goal is to stop him from gaining more ground and certainly to thwart his chances of becoming the Democratic 2020 nominee.

David Brock, a political consultant, told the Times: "There's a growing realization that Sanders could end up winning this thing, or certainly that he stays in so long that he damages the actual winner."

Brock told the paper that he has had discussions with other operatives about an anti-Sanders campaign and believes it should commence "sooner rather than later."

The Sanders campaign was not available for comment. The Democratic National Committee did not immediately respond to a request for comment.

Already, a fight has erupted between Sanders and the Center for American Progress, a think tank that has ties to Hillary Clinton.

But Uygur sees another reason for the establishment's concerns about Sanders: "The big Dem donors are not worried he's going to lose the general election, they're worried he's going to win," he tweeted on Tuesday.

Tweet

Uygur's point is that big money donors who back mainstream candidates will lose their positions of power in the face of a Sanders win. A political system corrupted by corporate cash and influence, he said, will finally fall if the democratic socialist who has proposed "Medicare for All," free college education and a $15 minimum wage is elected.

"The people with the most money will lose all power, they will not be rewarded," he said. "All their power is in giving money to politicians and controlling them. Bernie Sanders doesn't want their money. They are going to fight Bernie Sanders harder than any Republican will."

That could be a dangerous game: Attacking Sanders will "force everyone to rally around him," Uygur said. "That's why Bernie has always stood an excellent chance of winning. They can't take away a small donor base. The more they attack him, the more he will raise."

Indeed, the Sanders' campaign is already fundraising on the party conflict, dashing off a pitch this week that read in part: "The Democratic establishment and high-dollar donors are already planning how to stop our campaign. They are terrified of our movement - as they should be."

Ryan Grim, Washington bureau chief for progressive news site The Intercept, a former contributor to MSNBC and a contributor to the Young Turks Network also believes that establishment Democrats are afraid a Sanders win could upend the entire system they operate within.

"They couch that fear in claims that they're worried he'll lose," he told CNBC. "Change is scary."

Margo Baldwin, president of Vermont-based Chelsea Green Publishing, called the Democratic establishment "part of the power elite in bed with the banks and the moneyed interests." The company published "Rules for Revolutionaries" by former Sanders' campaign staffers Becky Bond and Zack Exley, as well as "Essential Bernie Sanders and his Vision for America" by Jonathan Tasini. "They don't like Trump, but they don't want to see real change," Baldwin added.

She joked that the party doesn't seem to like winning either, adding that they "can trot out Biden and they will lose."

But Markos Moulitsas, founder and publisher of liberal blog Daily Kos said it's not fear, "it's frustration" that party insiders are feeling. He said in an email that Sanders is "a divisive person" who "roils the party."

"As for the big donors, the best way to fight them is to build a small dollar machine that will power candidates at all levels of office," Moulitsas said. He added, "But even if we succeed in building an entire party of small dollar donors, weep not for big corporate money. They'll always have Republicans to buy."

It's certainly too early to tell where the race will end up, but for Sanders' supporters, a replay of the infighting that happened in 2016 will not go over well.

"There will be no end to the fury," Uygur said.