Alexandria Ocasio-Cortez’s endorsement was perhaps the most sought after in the race, but it is also something of a risk for the first-term congresswoman.

It was only 11 months ago when Alexandria Ocasio-Cortez shocked the political world with a primary upset of Representative Joseph Crowley, the No. 4 Democrat in the House and a mainstay in the Queens Democratic machine.

Then the youngest woman ever elected to Congress took Washington by storm, using her savvy social media skills to help achieve mainstream buy-in for ideas once deemed too far left for Democrats.

Now Ms. Ocasio-Cortez, 29, may be testing the extent of her influence back home: In a hotly contested seven-way Democratic primary for Queens district attorney, she has decided to endorse Tiffany Cabán, the candidate whose platform most echoes the first-term congresswoman’s.

“Our criminal justice system needs to change,” Ms. Ocasio-Cortez told The New York Times in a statement. “New Yorkers deserve a seat at the table, and a champion who will fight to realign our priorities toward equal treatment under the law. If Tiffany Cabán wins, things are going to change.”

The Democratic primary on June 25 is seen as something of a referendum on whether a diverse borough like Queens is ready to embrace the liberal criminal justice reforms — aimed at ending the era of mass incarceration — that are sweeping the country.

At the same time, Ms. Ocasio-Cortez may also get some sense if her win in the 14th Congressional District, which covers parts of Queens and the Bronx, can be extrapolated to a boroughwide race with more than one million registered voters.

Ms. Cabán, 31, is only the second person Ms. Ocasio-Cortez has endorsed since being sworn in. This month, Ms. Ocasio-Cortez endorsed Raul Fernandez, a candidate for the Brookline Select Board in Massachusetts.

Mr. Fernandez, a friend from her time attending Boston University, won a three-year term. Now Ms. Cabán is hoping an endorsement from Ms. Ocasio-Cortez will help distinguish her in a crowded field.

“We are proud of all the endorsements we have gotten, but this is a big deal,” Ms. Cabán, who previously worked as a public defender, said. “Alexandria Ocasio-Cortez is a progressive champion and she’s been an inspiration for me. It shows that we are building a national, grass-roots progressive movement.”

Ms. Ocasio-Cortez’s endorsement was one of the most sought after among progressive candidates in the race to replace the longtime district attorney, Richard A. Brown, who died May 4 from complications of Parkinson’s disease. Mr. Brown had previously announced he would resign on June 1, the 28th anniversary of his taking the office.

Ms. Ocasio-Cortez, who has more than 4 million followers on Twitter, is expected to send an email to her supporters this week announcing her endorsement and asking them to donate to Ms. Cabán.