WASHINGTON — The bad blood started early.

In 2008, Neera Tanden, then a top aide on Hillary Clinton’s first presidential campaign, accompanied Mrs. Clinton to what was expected to be an easy interview at the Center for American Progress, the influential group founded by top Clinton aides. But Faiz Shakir, the chief editor of the think tank’s ThinkProgress website, asked Mrs. Clinton a question about the Iraq war, an issue dogging her candidacy because she had supported it.

Ms. Tanden responded by circling back to Mr. Shakir after the interview and, according to a person in the room, punching him in the chest.

“I didn’t slug him, I pushed him,” a still angry Ms. Tanden corrected in a recent interview.

Ms. Tanden now leads the Center for American Progress, Mr. Shakir runs Bernie Sanders’s presidential campaign and the enmity between the two camps burst into the open last weekend. Mr. Sanders, angry about a video produced by ThinkProgress that ridicules his new status as one of the millionaires he has vilified on the campaign trail, sent a scorching letter to the center’s board, accusing Ms. Tanden of “maligning my staff and supporters and belittling progressive ideas.”

The blowup is another reflection of the ideological divisions among Democrats, this time between a legacy Clinton organization and a liberal wing trying to move the party to the left to harness the energy of millennials. Mr. Sanders’s team remains convinced that the Democratic establishment worked behind the scenes to deprive him of the party’s nomination in 2016; his campaign has cast the group as beholden to corporate interests set on thwarting him in 2020.