Supporters of Bernie Sanders' presidential campaign are railing against the "nervous" establishment as the Democratic presidential front-runner looks to battle off a mounting charge from Joe Biden as previous presidential candidates and other party leaders seem to coalesce behind the former vice president on the eve of Super Tuesday.

"[The] establishment is nervous not because we can't beat Trump but because we will," Sanders campaign manager Faiz Shakir told Fox News. "And when we do, the Democratic Party will again be a party of the working class."

This reaction from Shakir comes one day after former South Bend, Ind., Mayor Pete Buttigieg dropped out of the presidential race to endorse Biden and in the immediate aftermath of Sen. Amy Klobuchar's decision to do the exact same thing. Both are expected to appear at a rally with Biden Tuesday night.

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Former Senate Majority Leader Harry Reid, D-Nev., also announced he would endorse Biden Monday.

"Biden will be a much-needed stabilizing force following Trump's disastrous term, offering a positive and progressive alternate to Trump's dark vision of racism, xenophobia and policies built on cruelty and exclusion," Reid said in a statement. "I believe Biden is best able to defeat Donald Trump and enact policies we all care about."

Monday, Shakir on social media battled the idea that Biden was the best suited Democrat to fight Trump.

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"Anyone who thinks Biden would be a better candidate than Bernie against Trump hasn’t been watching the 10 Democratic debates," he tweeted. "Biden doesn't have the record, vision, excitement, coalition that Bernie does. We need to do more than just defeat Trump. We need progressive change."

He also panned Reid's Biden endorsement, calling back to an incident when Reid -- a specialist in legislative trench warfare -- tore up a list of concessions sent to him by the Obama White House during negotiations with Republicans, according to HuffPost.

For his part, Sanders tweeted an ad attacking Biden on his support for the Iraq War during the Bush administration. Biden, to be sure, voted in favor of the initial authorization of force that led to the war but says that the Bush administration mislead him on how far it would go with Congress' authorization.

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Outside of the Sanders campaign, former Green Party presidential candidate Jill Stein came after the Democratic establishment with an even sharper barb, citing former Democratic National Committee Chairwoman Debbie Wasserman Schultz's Sunday endorsement of Biden.

"Biden endorsed by Debbie Wasserman Schultz -- who stepped down from DNC in 2016 after she was caught sabotaging Sanders & elevating Trump," Stein tweeted. "How to lose to Trump again: pick another insider from the corrupt establishment with a lifeless campaign & long pro-war, pro-corporate record."

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Kyle Kulinski, a host of a show on the liberal The Young Turks network who supports Sanders, attacked both Biden and "the establishment" as he made a plea to Democratic voters to gather behind Sanders rather than Biden.

"The democratic [sic] establishment is so corrupt and scared of real change that they'd rather roll the dice on a guy with obvious cognitive decline, an abysmal policy record and no enthusiasm," Kulinski tweeted.

He continued: "If you believe in real change I'm humbly asking you to back Bernie tomorrow. You see how panicked the establishment is & how they'll stop at nothing to take him down. That should tell you something powerful," he said. "Yang people, Tulsi people, disaffected Warren people, Mayor Pete people who actually want change, non-voters, disaffected Trump people. All hands on deck. We have a real opportunity here."

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Biden, however, is riding high after what has been a strong two weeks for his campaign.

"I am very much alive!" he said during a Monday rally.

In an interview with Chris Wallace on Fox News Sunday, Biden said his Saturday win in South Carolina could be the start of something big.

“It’s a big boost,” Biden said. “I think it starts the real comeback and we picked up a lot of delegates.”

The Associated Press contributed to this report.