Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE is stirring things up in the homestretch of the Democratic presidential primary.

The Vermont Independent is threatening to take his challenge against Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE, whom he described over the weekend as the “lesser of two evils” compared to Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE, all the way to the Democratic National Convention in Philadelphia.

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He’s supporting Rep. Debbie Wasserman Schultz’s primary challenger in Florida, making his fight with the Democratic National Committee chairwoman personal.

On the legislative front, Sanders is intensifying his opposition to a Puerto Rico debt relief deal backed by Clinton, President Obama, House Democratic Leader Nancy Pelosi (Calif.) and even Rep. Raúl Grijalva (D-Ariz.), one of Sanders’s own supporters on Capitol Hill.

The multifronted fight is sparking worries among Democrats, who see Trump as winning time to unify the GOP as their own party remains engaged in internal war.

Democrats also see risks for Sanders; they warn the senator is undermining his credibility and influence going forward by arguing the system has turned against him.

“It’s a shame what’s happening now because Bernie Sanders is undermining the brand he was trying so hard to build,” said Cornell Belcher, who served as Obama’s pollster in 2008 and 2012 and has remained neutral in this year’s primary.

“When Bernie Sanders said the game is rigged because he’s not winning it and he’s going to go all the way to the convention and be the nominee, he’s falling into what everyone hates about politicians and politics,” Belcher added. “You’re opening the door to contention and fighting and people in rebellion because the process is illegitimate. That’s almost unforgivable.”

A spokesman for the Sanders campaign did not respond to a request for comment.

Sanders is behind Clinton in pledged delegates and total votes but has won nearly as many states as his Democratic rival. He’s run a competitive race against the former first lady and forced her to move to the left on issues such as trade to keep up with the Democratic base.

As a result, Sanders has increased his influence and power in Democratic politics, ensuring he’ll have some role in the Democratic platform.

Yet Democrats critical of Sanders see the Puerto Rico debate as an encapsulation of what they dislike about the Vermont lawmaker.

The legislation would create an outside fiscal control board to steer the territory’s finances, effectively putting in place a new body that would supersede Puerto Rico’s elected officials. Republicans insisted on the creation of the board as part of the deal to let the territory restructure its debt.

While Democrats aren’t thrilled about putting an unelected oversight board in place, they say something needs to be done and note legislation needs to pass the Republican-controlled House.

Sanders argued in a letter to Senate Democrats on Monday that the bill looks out for Wall Street first and foremost.

“That is unacceptable,” he wrote. “We have an important choice to make: do we stand with the working people of Puerto Rico or do we stand with Wall Street and the Tea Party? The choice could not be clearer.”

Former Rep. Barney Frank (D-Mass.), a Clinton supporter who has frequently criticized Sanders, said the senator’s Puerto Rico arguments invite comparisons to conservative Sen. Ted Cruz Rafael (Ted) Edward CruzVideo of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Democrat on Graham video urging people to 'use my words against me': 'Done' MORE (R-Texas), who has repeatedly criticized other Republicans for lacking purity by supporting various pieces of legislation.

“The question isn’t whether it’s good to have an oversight board or not. Ideally, I think the Democrats wouldn’t be pushing it,” Frank, the former chairman of the House Financial Services Committee, told The Hill in an interview Monday.

“The question is, if the price of any help for Puerto Rico in the current situation is an oversight board of some kind, what’s better?” he said. “It’s that kind of an approach — state an ideal, refuse to deviate from it in the interests of something that’s [doable] — it’s an explanation of why he has virtually no support from his colleagues.

“People talk about how Ted Cruz doesn’t have any support. Sanders doesn’t have any more either,” Frank said.

Sanders has far exceeded expectations by raising millions of dollars and giving Clinton a much tougher than expected challenge.

But his influence in Washington may be short-lived if he torches her and the rest of the Democratic establishment before ending his campaign, Democrats say.

Clinton supporters were outraged when Sanders’s wife, Jane, told Fox Business Network’s Neil Cavuto late last month that the FBI should speed up its investigation of Clinton’s use of a private email server.

Some Clinton backers think Sanders is stalling for time in hopes that a damaging FBI report will give him a chance to swoop in and grab the nomination at the July convention.