But he also faces another hurdle: hard feelings that remain to this day after the contentious 2016 Democratic primary. Many in the party continue to believe the Vermont senator played a role in contributing to Clinton’s defeat in November because of his criticisms of her prior to the general election, and his refusal to concede earlier when it appeared he had little mathematical chance of securing the party nomination.

And they are eager to point out Sanders’ flaws and examples of what they perceive to be examples of hypocrisy now that the one-time underdog rates as one of the front-runners in the crowded Democratic field.

“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,” said Zac Petkanas, who was the director of rapid response for the Clinton campaign.

In 2016, after Sanders endorsed Clinton and agreed to campaign on her behalf, the Sanders campaign’s preferred mode of travel quickly emerged as a point of tension, according to six former Clinton campaign staffers and another source familiar with the travel.

Those who had worked on his primary campaign made it known that the only logistical way Sanders could adhere to the event schedules requested by the Clinton campaign was by flying private. The Clinton campaign, however, viewed the private jet flights as a needless extravagance and wanted the senator to mostly fly commercial.

The senator ended up flying private on three separate multi-day trips in the last two months of the campaign.

Sanders’ flights — usually on a Gulfstream plane — cost the Clinton-Kaine campaign at least $100,000 in total, according to three people familiar with the cost of the air travel.

“We would try to fight it as much as possible because of cost and availability of planes, but they would request [a jet] every time,” one of those sources said. “We would always try to push for commercial. ... At the campaign, you’re constantly trying to save like 25 cents.”

Prior to working out the logistics of Sanders’ travel, “our working assumption was that 90 percent of the time it would be commercial,” said another person familiar with the matter. “If he was trying to hop from city to city in a particular state and [commercial] didn’t work, we were open to” chartering a plane.

But that idea did not go over well with the Sanders camp, according to this person.

“At that time, getting him on board — no pun intended —and his followers engaged for her, was a big priority,” said one former Clinton staffer, who explained that campaign leadership, including campaign manager Robby Mook, decided it was something Sanders wouldn’t budge on, so the campaign approved the requests to keep peace with the senator.

“We needed all hands on deck, including Bernie, and we were grateful for his support and the generous amount of time he gave the campaign,” Mook told POLITICO when asked for comment.

Sanders spokesperson Arianna Jones said it was physically impossible to get to all of the 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,” she said. “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.”

In the final three months before Election Day 2016, Sanders held 39 rallies in 13 states on behalf of Clinton’s campaign, according to Jones, including 17 events in 11 states in the last week alone. When he went to New Hampshire, which borders Sanders’ home state of Vermont, he did not use a private jet to get there.

Rania Batrice, who served as Sanders’ deputy campaign manager at the end of his 2016 campaign, said that Clinton’s campaign would send over a proposed campaigning schedule for Sanders before the two sides talked through logistics and “at no point did I ever say ‘he has to have a private plane for the sake of having a private plane.’”

“The requests for a charter only came after the schedules were put in front of us. If a less rigorous schedule were put in front of us, we wouldn’t have needed a charter and that would have been fine for everyone involved, including Bernie,” she said, later adding: “Bernie worked his ass off on behalf of Hillary Clinton and the campaign.”

Josh Orton, a Sanders aide, said, “The senator was proud to work tirelessly on behalf of Sec. Clinton, and we believe that we must put our past differences aside and unite to defeat Donald Trump.”

To the Clinton staff, however, the issue of the senator and the private jets became so cumbersome that it turned into “a running joke in the office,” said one former Clinton staffer.

The travel details weren’t the only point of tension with Sanders. At his rallies for Clinton, Sanders sometimes only wanted people who had endorsed him in the primaries to speak when he would appear, a request which frustrated the Clinton campaign, according to a former Clinton staffer and another person familiar with the matter.

“Sure you can have your supporters there, but you can’t exclude the congressman who endorsed Hillary Clinton in the city you are going to,” said the former staffer. “You’re campaigning for us. That was always a battle every single time.”

Batrice, who became a full-time DNC consultant for the general election, called that allegation “ridiculous” and said that the Clinton campaign actually blocked several people Sanders wanted from speaking at rallies that he was headlining, but she declined to name them.

Just as many former Clinton supporters and staffers continue to hold a grudge against the Vermont senator, many Sanders supporters still nurse grievances about the way the senator and his campaign were treated by the Clinton campaign and the party establishment, including the Democratic National Committee.

Michael Briggs, Sanders’ 2016 campaign spokesman who often traveled with Sanders on the private flights, said Clinton and her staff were “total ingrates” in light of the efforts the Vermont senator put in to try to help elect her in the general election.

“You can see why she’s one of the most disliked politicians in America. She’s not nice. Her people are not nice,” he said. “[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 assholes in American politics,” he added.

Private jet travel on the campaign trail is not uncommon — either for candidates like Clinton or a top surrogate tasked with stumping for them. Often it is the most efficient mode of transportation, particularly when events are in locations where commercial air travel options are limited.

One veteran Democratic operative who oversaw surrogates for past presidential campaigns said providing private planes is standard practice for the most important surrogate of a presidential campaign in the general election.

In addition to Sanders, the Clinton campaign footed the private plane bill on occasion for several top celebrities, among them Beyonce, Jay Z and Katy Perry. But as a rule, when political surrogates made requests for private jets, the campaign’s answer was no — except when it came to Sanders, said one former Clinton staffer.

The Clinton campaign itself spent a total of $15.9 million on jet charter company Executive Flightways in the 2016 campaign, according to a review of FEC records. That money was used to ferry Hillary Clinton, former President Bill Clinton, vice presidential nominee Tim Kaine, Sanders, major celebrities, and Clinton’s traveling press corps.

The revelation of Sanders’ penchant for private jet travel, both in 2016 and in the subsequent years, could surface as an issue for him since he often demands the U.S. do its part to fight global climate change — to which CO2 emissions from aviation is a contributor.

“Climate change is not only real, it is already doing irreparable harm all over this planet, including the United States of America,” he told CBS’ “Face the Nation” last November. “What Congress has got to do is take Trump on, take the fossil fuel industry on, and transform our energy system away from fossil fuel, to energy efficiency and sustainable energies like solar and wind.”

But last year, when Sanders faced reelection but faced only token opposition, his Senate campaign committee spent $342,000 on Apollo Jets, a private jet service. That money was used primarily to pay for a nine-day, nine-state tour to support Democratic candidates across the country.

Sanders held 18 “large public events in those days,” said Jones. “A review of airline flight schedules shows that this tour itinerary simply could not have been completed with commercial flights — it required a charter.”

She noted that the tour was the only time last year when Sanders flew on a private jet and also said the campaign purchased carbon offsets to zero out the emissions produced on the trip. Cars were also used on the ground whenever possible to get him to his events. She said that Sanders also flew commercial last year to hold more than 45 major rallies and events in 18 different states.

But the previous year, in April 2017, Sanders went on another far-reaching trip where private air travel was a significant component. According to a former DNC official and another person familiar with the details, Sanders’ camp requested a private jet for the “Come Together and Fight Back” unitytour with DNC chairman Tom Perez and other DNC officers — which included stops in Maine, Kentucky, Florida, Texas, Nebraska, Utah, Arizona and Nevada. The DNC paid Apollo Jets $157,000 that month, according to FEC records.

The next month, Sanders’ political committee transferred $100,000 to the DNC to share some, but not all, of the costs of the entire tour, which encompassed both the plane rides and other costs associated with the trips.

Asked whether Sanders will fly private for his 2020 campaign, Sanders spokesperson Jones said he “will be flying commercial whenever possible. 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.”

Scott Bland and Alex Thompson contributed to this report.