Bringing her political star power to races for Congress, Congresswoman Alexandria Ocasio-Cortez on Friday endorsed Democratic Senator Ed Markey for re-election in Massachusetts as he faces a potential primary challenge from Congressman 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." Markey is her Senate partner on the Green New Deal climate change proposal. He introduced the controversial bill in the Senate.

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

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"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 in the video. "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.

"He knows about the issues of our time, and he's not just resting on his record of the past, but he's aggressively pursuing an agenda for the future. And that's what progressivism is all about," Ocasio-Cortez said.

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

Markey's backing by the influential freshman lawmaker could pose a setback for Kennedy. Markey has been racking up endorsements while 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.

Kennedy filed a statement of candidacy and a principal campaign committee for a 2020 U.S. Senate run in August.

Markey has also been endorsed by his fellow Massachusetts senator, Elizabeth Warren, who is currently running for president. Kennedy, who was once taught by Warren at Harvard Law School, has been stumping on the campaign trail for his former professor.

Kennedy told reporters in September that he understood why Warren had endorsed Markey and that he was close to making his own decision.

"Primaries get messy. And I totally understand that. I respect her and the position of other folks in the delegation," he said, adding quickly, "That's not a major factor in my decision."

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 A.O.C., 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.

Nicole Sganga and Grace Segers contributed to this report