The other day, the junior senator of Vermont, Bernard Sanders, fired off a letter to the secretary of transportation, Peter Buttigieg, urging him “to take immediate action to substantially reduce the number of airline cancellations and delays” and calling the secretary’s handling of the crisis in our skies “simply unacceptable.”

Welcome to a budding feud between two potential contenders for the Democratic nomination for president — the aging Socialist of Vermont and the careerist at the Department of Transportation. It’s also a proxy war for the ideological divide in the party, with Mr. Sanders on the left and Mr. Buttigieg on the right.

Mr. Buttigieg has long been a progressive bete noire. A summer of discontent over air travel has presented fresh opportunities for progressives to criticize a man who once admired Mr. Sanders and who, like Mr. Sanders, has the White House on his mind. Their feud is echoing on the Democratic hustings.

Representative Ro Khanna, whose name has been bandied about as an heir to Mr. Sanders, has been carping about the travel situation. “This is embarrassing and should never happen in America,” he harrumphed. He suggests that Mr. Buttigieg needs to look at what President Obama did and “get their act together.”

That line echoes the letter sent by Mr. Sanders, who argued that Mr. Buttigiege’s department “could take cues from Obama era rules meant to prevent mass flight cancellations.” Mr. Buttigieg responded in an interview on CNBC, claiming he gave Mr. Sanders “a call” but had “not seen the math” on the senator’s proposals.

That lackluster riposte frustrated Mr. Buttigieg’s own surrogates, while Mr. Sanders’s staff director, Warren Gunnels, fired back, “I keep hearing that the transportation department imposed ‘record fines’ on airlines. I’m not aware of any fines imposed on airlines for failing to provide some $15 billion in refunds owed to passengers.”

In 2020, Mr. Buttigieg, who ran for president while mayor of South Bend, Indiana, finished second to Mr. Sanders in the New Hampshire primary. “How are we going to possibly fix our economy if the choice is between a socialist who thinks capitalism is the root of all evil and somebody who believes money is the root of all power,” Mr. Buttigieg said, referring to Mr. Sanders and Donald Trump.

In 2000, Mr. Buttigieg won a Profiles in Courage essay contest for his essay about Mr. Sanders. His essay contest was a homage to Mr. Sanders’s activism. 

“He and a few others like him have the power to restore principle and leadership in Congress and to win back the faith of a voting public,” Mr. Buttigieg said in reasoning out why Mr. Sanders is a political character of courage. While the admiration is still there, the pair’s relationship has become a rivalry.

Before the airline crisis, Mr. Buttigieg had been viewed as a potential figure to receive the torch from Mr. Biden were the president to eschew running in 2024. Mr. Buttigieg has the highest approval rating,  38 percent, amongst President Biden’s Cabinet, and is even with the commander in chief.

While Mr. Biden has said he plans to run for a second term, recent polling from Harvard indicates that only 30 percent of Democratic voters would vote for him if a competitive primary were held. Hence Mr. Sanders’s email asking supporters to demand that Mr. Buttigieg take action against the airlines. 

“Tell Pete Buttigieg to take action to reduce flight cancellations and delays in this country,” the campaign-style email sent from Mr. Sanders’s campaign email said. 

Mr. Buttigieg could be grilled by Congress if the flying situation continues to deteriorate.  Mr. Sanders, who chairs the Senate Budget Committee, and Mr. Khanna, a member of the House Oversight Committee, could both subpoena Mr. Buttigieg to testify in the coming weeks about his handling of the crisis. 

“Taxpayers,” Mr. Sanders wrote to his former admirer, “bailed out the airline industry during their time of need.  Now, it is the responsibility of the airline industry and the Department of Transportation to ensure, to the maximum extent possible, that the flying public and crew members can get to their destinations on time and without delay.”