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.