Elizabeth Warren

U.S. Sen. Elizabeth Warren endorsed Daniel Lander, a former senior advisor to Boston Mayor Michelle Wu vying to unseat state Sen. William Brownsberger, who has drawn the ire of Wu since helping to kill her tax shift bill.

Warren, an ally of Wu whom the mayor counts as a mentor, is endorsing against a member of state Senate leadership for the first time, according to the Lander campaign, which announced Warren’s endorsement on Thursday.

Brownsberger is the president pro tempore of the Massachusetts Senate.

The Lander campaign characterized Warren’s decision to break ranks as a “clear” sign that “Lander’s campaign has serious momentum in its fight to challenge Senator Brownsberger, the architect of the broken status quo on Beacon Hill.”

“I know firsthand that Daniel is a fighter who is ready to make change for the people of Massachusetts,” Warren said in a statement. “He’s an organizer, a problem solver, and has an impressive track record of public service. I have no doubt that Daniel will hit the ground running to lower housing costs, upgrade our infrastructure, and make a real difference for working families.”

Before working as a senior aide to Mayor Wu, a job his campaign said he left in January to focus on the Senate race, Lander worked as a senior political advisor for Warren’s 2020 presidential campaign. His work involved leading national organizing of LGBTQ+ and youth communities, per the Lander campaign.

Lander said he is “deeply honored to earn the endorsement” of Warren, calling her a “political hero” who has inspired him since her first Senate run in 2012.

“When Senator Warren first challenged an incumbent senator, she faced an uphill battle and won by challenging a broken status quo that was leaving too many behind,” Lander said in a statement. “Senator Warren fights every day to challenge the broken status quo in Washington to deliver for working people.

“I am running against the architect of the failing status quo on Beacon Hill, and I look forward to fighting alongside her in Massachusetts to make sure everyone can build a good life in our communities,” Lander added.

Lander has also worked with Stacey Abrams, a former member of the Georgia House of Representatives and a progressive Democrat like Warren and Wu.

His campaign said he is running for the Suffolk and Middlesex Senate district seat to address the region’s housing crisis, improve the MBTA, and “stand up to the Trump administration.”

The high-profile Warren endorsement for Lander follows Wu’s backing two weeks ago of Latoya Gayle, a political newcomer who is challenging state Sen. Nick Collins, another Boston senator who joined Brownsberger in leading the effort to kill Wu’s signature tax shift bill on Beacon Hill.

The bill seeks to provide a tax break to homeowners by increasing commercial tax rates. It was passed by the City Council and House of Representatives three times, but stalled in the Senate.

Brownsberger in March was endorsed by Gov. Maura Healey, who is also running for reelection.

“Senator Brownsberger’s leadership has been a crucial part of so much of our state’s progress in the last decade, and we need him back on Beacon Hill next year,” Healey said in a statement at the time. “I’m grateful for his partnership, and I’m proud to support his reelection.”

Brownsberger, a Belmont Democrat who represents parts of Boston, has been on Beacon Hill for nearly two decades, first as a member of the House of Representatives. He won his Senate seat in a 2011 special election, and until this year, had not faced a challenger for his current office.

The Democratic primary for state Senate is Sept. 1.