Chi Ossé, a far-left city councilman aligned with the policies of Zohran Mamdani, the mayor-elect of New York City, has told political allies that he is planning to challenge Representative Hakeem Jeffries, the top House Democrat, in next June’s primary.

But he may not have the mayor-elect’s support.

Mr. Mamdani has privately tried to discourage his ideological ally from running. The mayor-elect and his team fear that another high-profile challenge from the left might compromise his own bid to push the Democratic establishment to support his affordability agenda.

The disagreement caused Mr. Ossé to be disinvited from Mr. Mamdani’s election night watch party, according to two people familiar with the matter, even though he has been a frequent presence at Mr. Mamdani’s campaign events. Instead, Mr. Ossé said he spent the night “with my constituents at D.S.A. events celebrating Zohran’s incredible win.”