As the Democratic runner-up in 2016, Bernie Sanders has a built-in advantage that’s vaulted him to the front of the 2020 pack. He has nearly unparalleled name recognition, raised nearly $6 million within 24 hours of announcing his White House bid, and enjoys a sizable polling lead over several top rivals. Of course, front-runner status also makes Sanders a target—and there’s no shortage of opposition research his detractors are happy to surface. Last month, a group supporting Beto O’Rourke released a video of Sanders during a trip to the Soviet Union, drinking and shirtless, singing the Woody Guthrie song “This Land Is Your Land.” More recently, a video has been recirculated in which Sanders, after returning from the Evil Empire, praises the U.S.S.R.’s breadlines.

Few, however, have been as vocal, or as snippy, as Hillary Clinton’s former supporters, who took the time to tell Politico that they just cannot stomach the Vermont senator’s hypocrisy. Why? For having the audacity to request the use of a (carbon-spewing) private jet so that he could crisscross the country campaigning for her.

Team Clinton’s criticism rests on Sanders’s anti-fossil fuel agenda, which in some respects is even more ambitious than Alexandria Ocasio-Cortez’s Green New Deal (which seeks to reduce air travel by investing in high-speed rail). While bashing environmentalists for their travel habits is a time-tested practice, one rarely hears it from a liberal to attack a fellow liberal—unless, of course, it’s Clinton supporters feasting on old sour grapes. “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 Clinton’s former director of rapid response, Zac Petkanas.

Others were less eager to put their names on the record, but took potshots nonetheless:

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.