WASHINGTON — House Democratic leaders, their patience clearly fraying, signaled this weekend to Representative Alexandria Ocasio-Cortez’s outspoken top aide that his seeming efforts to lead an insurrection against more moderate Democrats would no longer be tolerated — a message also aimed at the freshman congresswoman who employs him.

Ms. Ocasio-Cortez, a liberal firebrand from the Bronx, has given her chief of staff, Saikat Chakrabarti, remarkable latitude to pursue the divisive politics that made his name when he led Justice Democrats, a group founded to challenge entrenched Democrats through primary campaigns.

With that license, Mr. Chakrabarti has become an unelected symbol of the party’s growing disunity, as Democrats try to coalesce as a party before what promises to be a punishing fight next year for the White House. The battle between the Democrats who secured the House majority last year by flipping Republican districts and the smaller, but politically potent, left-wing from secure Democratic districts has found its cause célèbre.

Mr. Chakrabarti ignited a firestorm two weeks ago after a bruising intraparty fight over an emergency border aid package that progressives said lacked sufficient restrictions on the Trump administration. Calling out moderate Democrats who sank a more liberal aid package, he compared them to “new Southern Democrats.”