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{shortcode-69a9ed06c887cb075e6988b5c6d61980cc21c96c}inutes before she was slated to speak at a candidate forum hosted by the Cambridge Democratic Party Thursday evening, incumbent Massachusetts State Rep. Marjorie C. Decker listened as the crowd, bearing campaign signs and t-shirts, chanted her name in roaring waves.

The sea of supporters served as a reminder of Decker’s strength and a testament to the deep network of backers she developed over her 25-year career in politics, beginning with her election to the Cambridge City Council in 1999.

But the forum itself was also a bitter reminder of how Decker’s grip over her home city has loosened as she enters the final days of a bruising primary fight against a left-wing challenger that has forced her legislative record into the spotlight.

Her opponent in the Sept. 3 Democratic primary is Evan C. MacKay ’19, a staunch progressive and former Harvard labor leader who has formed the most potent and well-funded threat to Decker’s candidacy since she was elected representative for the 25th Middlesex district in 2012.

Decker, whose district encompasses Harvard’s Cambridge campus, sits on a six-figure pile of campaign cash and boasts endorsements from political heavyweights like Governor Maura T. Healey ’92. But successful fundraising and a potent message around government transparency have turned MacKay, who previously led the Harvard Graduate Students Union-United Auto Workers, into a serious threat for the longtime incumbent.

In less than six months, the once-quiet district has been turned on its head as hundreds of thousands of dollars in spending pour in and make the race into among the most hotly contested in the state.

Here’s what you need to know about the race ahead of Tuesday’s elections.

Old Guard, New Blood

As the Sep. 3 primary nears, the contest between MacKay and Decker has quickly become a referendum between Massachusetts’ political establishment and a younger, left-wing insurgency.

MacKay has tied Decker to the state’s political establishment, instead offering themself as a progressive alternative and catalyst for change.

“I want Cambridge to do even more to tax corporations and the rich. I want Cambridge to do even more on climate policy,” MacKay said. “Our state is really well-positioned there.”

Their message has resonated with the state’s progressive wing, securing an endorsement from Jivan Sobrinho-Wheeler, the foremost progressive stalwart on the Cambridge City Council.

MacKay is also receiving support from pro-transparency group Act on Mass, advocacy group Cambridge Bike Safety, and the Cambridge and Boston chapters of the Democratic Socialists of America, lending the insurgent campaign crucial organizing muscle as the race enters its final stretch.

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But Decker has seized on the opportunity to defend her record, highlighting her work on health care reform as the chair of the powerful Joint Committee on Public Health. She has also crowed about steering millions of dollars in state funding toward Cambridge for infrastructure and other local projects.

And she has relied on her connections, winning coveted endorsements from Healey, Sen. Ed Markey (D-Mass.), Rep. Ayanna M. Pressley (D-Mass.), and five of the nine members of the City Council, including Mayor E. Denise Simmons and Vice Mayor Marc C. McGovern.

Decker is also endorsed by Planned Parenthood of Massachusetts, the Mass. Women’s Political Caucus, and the statewide chapter of the AFL-CIO. The Boston Globe endorsed her in a Tuesday editorial.

Decker, already entering the race on a mountain of campaign cash — from May 2021 to January 2024, she never reported less than $100,000 on hand — has maintained her financial advantage over its course. Her fundraising has vastly outperformed MacKay’s.

But MacKay’s challenge has forced Decker on the defensive, and to spend more than she ever has against a primary opponent. In July alone, she spent over $42,000 — the most she has ever spent in one month throughout her entire career in Massachusetts politics.

Transparency Takes the Limelight

Throughout the campaign, MacKay has cast Decker as a stand-in for Beacon Hill’s lower chamber, the State House, which has long faced criticism for its opaque legislative processes and a committee system which allows members to vote in effective secrecy.

Decker has voted against amendments to House rules aimed at making lawmakers’ voting records more transparent, including 2017 and 2019 attempts by State House Republicans to mandate that committee votes be published on the State House website.

Both amendments failed overwhelmingly amidst opposition from Democratic leadership.

In the Thursday forum, Decker appeared to reverse her position, saying that committee votes “should be online.”

Still, MacKay has continued to attack Decker for her history on the issue.

“We’ve done letters to the editor. We’ve met with her office, we’ve lobbied her,” MacKay said of Decker in an interview with The Crimson. “She has been a staunch opponent of the needed reforms within the State Legislature towards anti-corruption and transparency.”

In recent days, Decker’s personal dealings have also been the subject of scrutiny. She took in at least $800,000 between 2016 to 2023 from Boston law firm Berman Tabacco, according to financial disclosures first reported by the Cambridge Day — income that neither Decker nor the firm had previously discussed publicly.

Still, on the campaign trail, Decker has fiercely defended her record as a legislator.

In an early August mailer, Decker wrote that she aims “to be a responsive and accessible resource that people feel comfortable reaching out to” — highlighting her constituent newsletter and office hours, as well as publicly available resources to search legislation and view committee hearings online.

In a statement to The Crimson, Decker wrote that she is “a staunch advocate for transparency, accountability and honesty” and that she takes pride in “upholding good government.”

“I agree wholeheartedly that the legislature has an obligation to provide to the citizens of the Commonwealth information about their work,” she wrote.

An Entrenched Incumbent

Before MacKay’s run, Decker proved adept at firmly deflecting primary challengers — in large part because of her own rapid rise across Cambridge politics.

She became the youngest woman ever elected to the Cambridge City Council, running on a campaign that emphasized her roots in Cambridgeport’s public housing.

“My campaign really strove to bring different people together on the issues,” she told The Crimson shortly after her victory at the time.

She set her sights on state office soon after, mounting a 2002 primary challenge of her own to then-incumbent State Rep. Paul C. Demakis, whose district was split between Cambridge and Boston.

Originally considered a long shot, it was her well-oiled campaign machinery — alongside endorsements from local favorite sons Matt Damon ’92 and Ben Affleck — that secured Decker 40 percent of the vote, just three years after her entrance to the Cambridge political scene. She ran again and lost eight years later, this time for an open seat in the State Senate.

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In 2012, however, State Rep. Alice Wolf — Decker’s former boss, and a longtime sponsor of her career — retired. With Wolf’s backing, Decker glided to the Democratic nomination and, later, handily won the general election the redrawn, deep-blue district.

Since then, Decker has repelled three primary challenges. In all cases, she never dipped below 83 percent of the vote.

Meanwhile, MacKay’s run for the 25th Middlesex is their first foray into elected politics. They are a Harvard graduate student who, until entering the race, served as president of Harvard Graduate Students Union-United Auto Workers — the union representing thousands of graduate students across the University.

During MacKay’s time in HGSU-UAW leadership, the union campaigned for the University to raise graduate student stipends to a “living wage” of $48,779 and sought mid-contract negotiations over graduate student salaries.

Harvard answered the union’s demands in Dec. 2023 — less than a month after MacKay departed the HGSU-UAW presidency to launch their bid for office — by announcing that doctoral students in the Graduate School of Arts and Sciences will receive stipends worth at least $50,000.

During her closing remarks at the forum, Decker stressed that she was a familiar face for Cambridge.

“Transparency means every day, being in the coffee shop, being on a softball field, being on the basketball court, being in the grocery store, being with you in community and answering all the questions that you have,” she said.

Later, during MacKay’s closing, they said their experience as a labor organizer would help them fight for — and alongside — their constituents.

“We have to recognize that a lot of the solutions rely with everyday people, and that our government should be accountable to everyday people,” they said.

But the applause by that point had become more tepid. Decker had already worked the crowd.

—Staff writer Matan H. Josephy can be reached matan.josephy@thecrimson.com. Follow him on X @matanjosephy.