WASHINGTON — Bringing her political star power to races for Congress, Rep. Alexandria Ocasio-Cortez on Friday endorsed Democratic Sen. Ed Markey for re-election in Massachusetts as he faces a potential primary challenge from Rep. Joe Kennedy III.

Ocasio-Cortez said in a video released by Markey's campaign that she's backing the Democratic incumbent as one of the Senate's "strongest progressives" and her partner on the Green New Deal climate change proposal.

She said she is "proud to enthusiastically support" Markey's reelection.

We have to do more than take back the WH.



We must do so w/ a stronger Democratic Party that fights for working people - & that includes newcomers+longtime fighters.



I’m proud to endorse my #GreenNewDeal co-author + climate champion @EdMarkey for Senate.pic.twitter.com/WrYmTaJrnn — Alexandria Ocasio-Cortez (@AOC) September 13, 2019

"When I first got to Congress and we started to discuss big, bold plans -- a solution on the scale of the crisis -- many members shied away," the freshman lawmaker said. "Ed Markey was one of the few people that had the courage to stand up and take a chance." The Boston Globe first reported the endorsement.

Backing from the influential freshman lawmaker is a potential setback for Kennedy, as Markey has been racking up endorsements and the congressman weighs whether to jump in the race.

A primary in Massachusetts would pit Markey, the longtime lawmaker, against a popular four-term congressman from the iconic political family. He is the grandson of Robert F. Kennedy.

Ocasio-Cortez's endorsement comes as the liberal newcomer is turning her political attention on congressional races for the House and Senate.

Earlier this week, Ocasio-Cortez said she's skipping presidential primary endorsements, for now. Instead, she told reporters she's focusing on House and Senate races. She said she'll be rolling out endorsements "soon."

"I really want to focus on our congressional endorsements first," the New York Democrat said this week.

"Our presidential candidates are fabulous, but they take up a lot of attention and so they don't need my help right now," she said. House and Senate races, she said, "really need a lot of help and attention."

Ocasio-Cortez is heading to Colorado next week to headline a fundraising dinner for Democrats in Boulder and participate in activities around the climate strike protests, a worldwide walkout of young people from schools, homes, jobs to demand action on climate change. She also said she would be holding a town hall with a Colorado lawmaker.

Known by her initials AOC, the liberal newcomer toppled a House Democratic leader with a remarkable 2018 primary challenge that stunned Washington.

But putting her political power to work is not without its risks. Lawmakers resent challenges from their peers in primary elections, and some are already warily watching her moves.

A group aligned with Ocasio-Cortez, Justice Democrats, has already announced its support for primary challenges to other congressional Democrats in 2020.