Bernie Sanders' 2016 presidential campaign spokesman unloaded on Hillary Clinton and her team on Monday, calling them the "biggest a--holes in American politics," after former members of Clinton's campaign leaked details this week about Sanders' use of private jets to attend campaign rallies on her behalf.

Speaking to Politico, the spokesman, Michael Briggs, proceeded to describe Clinton's staff as "total ingrates," given that Sanders claims he billed the Clinton-Kaine campaign for private air travel in order to attend events that he otherwise would have needed to skip.

“You can see why she’s one of the most disliked politicians in America," Briggs said, referring to Clinton. "She’s not nice. Her 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.”

Briggs added: “It doesn’t make me feel good to feel this way but they’re some of the biggest a--holes in American politics."

"She’s not nice. Her people are not nice." — Bernie Sanders 2016 spokesman Michael Briggs

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Several former Clinton staffers, also speaking to Politico, reported that Sanders' frequent requests for private planes from the campaign became “a running joke in the office" -- in part because Sanders is a socialist, and also because he has pushed for the elimination of carbon-generating heavy aircraft in favor of high-speed rail networks. In all, Sanders reportedly billed the Clinton-Kaine campaign approximately $100,000 for air travel.

Some bad blood remains between the Clinton and Sanders camp, according to insiders, in part because of Sanders' harsh criticisms of Clinton during the 2016 Democratic presidential primary.

Talking to the liberal “Pod Save America” podcast in 2017, Clinton said she "couldn't believe" that, because of Sanders, she was forced into "basically defending President Obama in a Democratic primary." And in her book, the election retrospective "What Happened," Clinton slammed Sanders' ideas as unrealistic and decried him for using “innuendo and impugning my character” such that she suffered “lasting damage" into the general election.

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Sanders spokesperson Arianna Jones, though, maintained that Sanders put everything he had into helping Clinton once she had secured the Democratic nomination. Jones said it was physically impossible for Sanders to get to all of the Clinton event locations in such a short period of time without chartered flights, especially since the senator was traveling to many smaller markets with limited commercial air travel options.

“That’s why chartered flights were used: to make sure Sen. Sanders could get to as many locations as quickly as possible in the effort to help the Democratic ticket defeat Donald Trump,” Sanders spokeswoman Arianna Jones told Politico. "Sen. Sanders campaigned so aggressively for Secretary Clinton, at such a grueling pace, it became a story unto itself, setting the model for how a former opponent can support a nominee in a general election.”

Jones reported that in the three months prior to the November 2016 election, Sanders supported Clinton by attending 39 rallies in 13 states.

Sanders stunned the Democratic establishment in 2016 with his spirited challenge to Clinton, and his campaign helped lay the groundwork for the leftward lurch that has dominated Democratic politics in the era of President Trump.

Within a week of launching his 2020 bid, the campaign has reported raising $10 million -- and more than $4 million of that in the first 12 hours after announcing. Previously, the biggest first-day fundraiser in the race had been California Sen. Kamala Harris, who raised $1.5 million in the first 24 hours of her campaign. And this week, Sanders announced he has already signed up a historic 1 million volunteers.

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The question now for Sanders is whether he can stand out in a crowded field of Democrats who embrace many of his policy ideas and who are newer to the national political stage -- and whether Sanders can survive with the evident lingering resentment from members of the Democratic Party establishment.

"Our campaign is not only about defeating Donald Trump," the 77-year-old self-described democratic socialist said in an email to supporters announcing his run. "Our campaign is about transforming our country and creating a government based on the principles of economic, social, racial and environmental justice."

As for whether Sanders -- who has pushed for the Green New Deal, which would strive to greatly reduce air travel -- would be flying commercial for upcoming campaign trips this year, Jones told Politico he "will be flying commercial whenever possible," and that the "campaign will consider the use of charter flights based on a variety of factors, including security requirements, logistics, and media interest in traveling with the senator.”

Also causing headaches for Sanders' socialist, penny-pinching image: His high-end income and multiple houses. Notably, he owns three houses. In 2016, he bought a $575,000 four-bedroom lake-front home in his home state. This is in addition to a row house in Washington D.C., as well as a house in Burlington, Vermont.

“The Bern will keep his home in Burlington and use the new camp seasonally,” Vermont’s Seven Day’s reported in 2016.

Fox News' Adam Shaw and The Associated Press contributed to this report.