WASHINGTON — Representative Alexandria Ocasio-Cortez plans on Tuesday to announce her endorsement of Marie Newman, a progressive candidate seeking to oust Representative Daniel Lipinski, a conservative-leaning Illinois Democrat, marking her first move of the 2020 campaign cycle to back a primary challenger to an incumbent.

Ms. Ocasio-Cortez’s high-profile support amounts to a powerful seal of approval, telegraphed to her legions of ardent liberal fans, on behalf of Ms. Newman, and a reflection of the zeal of the party’s progressive left to leverage its nascent power to continue targeting sitting Democrats.

Ms. Newman is backed by Justice Democrats, the group that propelled Ms. Ocasio-Cortez’s insurgent campaign and upset victory last year over a veteran Democratic congressman who she argued was out of step with his constituents. A businesswoman who has described herself as “a real Democrat,” Ms. Newman also ran in 2018 against Mr. Lipinski and lost by about 2,000 votes.

“Marie Newman is a textbook example of one of the ways that we could be better as a party — to come from a deep blue seat and to be championing all the issues we need to be championing,” Ms. Ocasio-Cortez said in an interview.