Bernie Sanders’ enduring popularity across Vermont for decades has scared off political challengers, but the Independent senator is facing competition in his 2018 re-election bid from a Democrat who thinks his “Robin Hood shtick” must end.

“It’s shamefully arrogant when you’re more interested in being a celebrity than honoring your progressive agenda,” challenger Jon Svitavsky told Fox News. “This wonderful, political ‘I am Robin Hood shtick’ can only last for so long.”

The longshot bid comes from a first-time candidate even more anti-establishment than the democratic socialist incumbent and, in his words, "far more liberal."

An advocate for the homeless who claims to have some name recognition in the state, Svitavsky not only questions the sitting senator's commitment to Vermont voters but argues he used and undermined the Democratic Party for his 2016 presidential bid.

'He’s not a Democrat. That was a joke.' — Jon Svitavsky, on Sanders' White House bid

Svitavsky contends Sanders joined Democrats to seek their nomination, then damaged front-runner Hillary Clinton enough to give then-candidate Donald Trump the edge in the general election -- only to once again become an Independent.

He also suggests an FBI investigation into a commercial real estate loan orchestrated by the senator’s wife, Jane Sanders, has left Sanders vulnerable.

FBI PROBE OF SANDERS' WIFE BASED ON 'FACTS,' GOP OFFICIAL SAYS

While his curmudgeonly manner has long alienated Capitol Hill colleagues, Sanders, a self-styled champion of the poor and middle class, continues to be immensely popular among voters.

A Morning Consult survey released Tuesday showed him with the highest approval rating among all 100 senators, 75 percent, based on interviews with registered voters in their respective states.

However, the poll was conducted from early April to mid-June, largely before reports of the loan started attracting national attention.

The federal investigation apparently focuses on whether Jane Sanders, as president of the now-shuttered Burlington College in Vermont, overstated or overpromised financial pledges and grants to get at least $6.7 million in financing in 2010 for roughly 33 acres for a new campus.

There also have been unsubstantiated allegations that Sanders, now seeking a third Senate term, used his political office to either get the loan approved or at least OK’d swiftly.

The senator, in various interviews, has called such claims an "absolute lie" while describing the criticism of his wife as "pathetic" and political.

The self-described democratic socialist also has more than $3.8 million cash on hand in Senate accounts, according to OpenSecrets.org, which only adds to Svitavsky’s complications.

Svitavsky hopes to win the state’s Democratic primary and challenge Sanders in the general election.

In an interview Tuesday, he sounded undeterred by Sanders’ popularity and war chest, saying that his decades-long efforts in opening homeless shelters across the state has given him standing among voters.

“I think that resonates,” he said. “And I’m not unknown here. People might say I don’t have political experience but not that I’m insincere. … I’m far more liberal than Bernie, far more committed to making things happen.”

Svitavsky says he's getting strong grassroots support from across the state and country -- including a call from a guy who used to play with folk-singing legend Pete Seeger.

The Sanders campaign has declined to comment on Svitavsky’s bid.

Before Sanders was elected to the Senate in 2006, he served 19 years in the House and eight as mayor of Burlington. He was re-elected to the Senate in 2012 with 71 percent of the vote, as proof of his political strength.

There has been no indication that the 75-year-old Sanders intends to retire before next year. And one source close to his 2016 presidential campaign recently told Fox News he wants to run for president again in 2020.

Right now, Sanders is still among the leading voices for national Democrats, even taking the spotlight from Democratic National Committee Chairman Tom Perez during a recent, multi-state tour that attempted to bridge the party’s lingering Clinton-Sanders divide.

“He’s not a Democrat. That was a joke,” said Svitavsky, who argues the Democratic Party was outfoxed by Sanders but is now coming to its senses. Svitavsky cannot officially file to run until next spring.

Sanders and his White House bid captured the political interests of tens of millions of voters -- particularly younger Americans -- with promises of a free college education, universal health care and legalized marijuana.

However, Svitavsky largely dismissed those promises as unrealistic because they would be too expensive for taxpayers, even if Congress approved them.

“This cannot go on forever,” he said.