Vermont Sen. and 2020 Democratic presidential hopeful Bernie Sanders has re-upped his fight against climate change and the wealthy, taking aim at the fossil fuel industry in the announcement of his new campaign.

However, bad blood over Sanders' perceived resistance in conceding to 2016 presidential nominee Hillary Clinton has some within the party speaking up about the socialist senator's own elitist habits.

Six former staffers from Clinton's failed 2016 presidential campaign are lashing out at "his Royal Majesty" Sanders, criticizing his frequent use of private jets when he traveled the country rallying Democrats to show up and vote for Clinton.

What are the details?

In an interview with Politico, Clinton's former campaign director of rapid response, Zac Petkanas, didn't mince words:

I'm not shocked that while thousands of volunteers braved the heat and cold to knock on doors until their fingers bled in a desperate effort to stop Donald Trump, his Royal Majesty King Bernie Sanders would only deign to leave his plush D.C. office or his brand new second home on the lake if he was flown around on a cushy private jet like a billionaire master of the universe.

A second Clinton aide told the outlet, "We would try to fight it as much as possible because of the cost and availability of planes, but [the Sanders camp] would request [a jet] every time. We would always try to push for commercial. ... At the campaign, you're constantly trying to save like 25 cents."

Another unnamed staffer added that initially, Clinton aides were surprised by Sanders' demands for private charters, saying, "our working assumption was that 90 percent of the time it would be commercial."

But soon, the senator's alleged insistence on being supplied with his own jet became "a running joke" in the Clinton campaign office, Politico reported.

How did the Sanders campaign respond?

Sanders' 2016 campaign spokesman, Michael Briggs, fired back at the Clinton camp, calling them "total ingrates" for dismissing the senator's efforts to help Clinton become president.

"You can see why she's one of the most disliked politicians in America," Briggs told Politico. "She's not nice. Here people are not nice. [Sanders] busted his tail to fly all over the country to talk about why it made sense to elect Hillary Clinton and the thanks that [we] get is this kind of petty stupid sniping a couple years after the fact."

"It doesn't make me feel good to feel this way but they're some of the biggest a***oles in American politics," he added.

Anything else?

Sanders' use of private jets has come under fire before, seen as hypocritical given the senator's platform against the use of fossil fuels. But he doesn't appear to be letting his agenda get in the way of his travel habits.

In October, Sanders' 2018 campaign paid just under $300,000 to a charter jet company, Fox News reported.

The Sanders campaign reportedly purchased less than $5,000 in carbon offsets to compensate for the emissions expelled by the senator's plane usage that month.