Dave Cavell, a former Obama speechwriter running in the Fourth Congressional District, dropped out of the congested Democratic primary on Thursday and quickly endorsed Jesse Mermell, a similarly progressive candidate, setting off a series of moves in the once-sleepy primary.
The surprise exit, announced just 19 days before the Sept. 1 primary and with mail-in voting already underway, is also strategic.
Cavell, 36, said he is backing Mermell, a former Brookline selectwoman, in an attempt to avoid splintering the vote in the now eight-person field and to block Jake Auchincloss, a Newton city councilor and former Republican and state GOP employee who has polled near the top of other campaigns’ internal surveys.
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“Jake is the elephant in the room — pun intended,” Cavell said Thursday. “One thing that has been abundantly clear is there is one candidate in this race who should not represent the Fourth District.”
A wave of developments quickly followed. Attorney General Maura Healey, whom Cavell had worked for up until last year, announced her own endorsement of Mermell hours later, giving the Brookline Democrat the formal backing of one of the state’s leading progressive political stars.
And shortly after that, a pair of super PACs — including one with the backing of SEIU and the Massachusetts Teachers Association — unveiled their own package of advertising in support of Mermell, including putting roughly $50,000 behind a television ad touting her as a “true blue” progressive.
The rapid-fire maneuvering had the potential to shift the race for Representative Joseph P. Kennedy III’s open seat in its waning weeks. Long overshadowed by Kennedy’s own challenge of Senator Edward J. Markey, the Fourth District contest has transformed from a polite, intraparty affair into a contentious primary, buffeted by the rapid rise of several outside group trying to influence voters.
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Auchincloss, for one, has become a frequent target of other candidates after he earned the endorsement of the Globe’s editorial board. He underwent an unusual public vetting this week in which he apologized for controversial past comments and social media posts, and with Cavell’s exit, appeared even more at loggerheads with the primary’s more progressive faction.
Auchincloss’s campaign brushed off Cavell’s comments Thursday as “political back-and-forth,” instead pointing to Auchincloss spending a part of the morning in Fall River with two state lawmakers and the city’s mayor, who have endorsed him, packing food at the local Boys and Girls Club.
“Jake is focused on the real issues impacting voters across the Massachusetts Fourth,” spokeswoman Yael Sheinfeld said.
Cavell, who left his job as an adviser to Healey to launch his congressional run last October, campaigned on his experience in the White House and support for progressive priorities, such as tuition-free public colleges.

But the Brookline Democrat had remained locked within the middle of the nine-person field. He raised about $400,000 over eight months, including $29,000 he loaned his campaign, while other candidates launched expensive television advertising campaigns or drew help from outside groups, giving them wider exposure to voters.
There has been no independent polling of the field. But an internal survey conducted for the campaign of Ihssane Leckey, and first released this week to the Intercept, showed Cavell polling in fifth place at 7 percent, just behind Mermell, Leckey in third, Auchincloss, and Becky Walker Grossman, another Newton city councilor who led at 19 percent.
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Given the timing of his announcement, Cavell’s name will remain on the ballot. But his departure could add some clarity to a field where six of the candidates hail from Brookline, a progressive stronghold. He was already aligned on several issues with Mermell, a one-time aide to former governor Deval Patrick who backs Medicare for All and supports the Green New Deal.
“I got into the race for the same reason I’m getting out. Because I care about the issues,” Cavell said. “I do believe that Jesse can be the progressive fighter for this district.”
In her own statement, Healey did not cite Cavell’s exit for backing Mermell. But Cavell discussed suspending his campaign with her before his announcement, and his decision “made her choice even clearer” to back Mermell, according to a Healey aide, who was granted anonymity to discuss her rationale.
“This was about Jesse, and her support for Jesse,” the aide said.
Healey and Mermell share political advisers, including the consulting firm Melwood Global, and the two worked together on reproductive rights issues when Mermell was vice president of the Planned Parenthood League of Massachusetts and Healey headed the attorney general office’s civil rights division, campaign aides said.
Such political jockeying is not out of character for Healey. The Charlestown Democrat, who doesn’t live in the Fourth District, waded into the 2018 Democratic primary between Ayanna Pressley and then-incumbent congressman Michael Capuano, endorsing Pressley in her upset victory.
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Her presence now in the Fourth is only the latest attempt to reshape the race. A rebranded super PAC that was once heavily funded by labor groups backing Mermell disclosed nearly $25,000 in spending on mailers supporting her.
The group, now known as Commonwealth Values PAC, first emerged as the Rethink PAC during the 2012 cycle, when it spent more than $1.1 million opposing then-senator Scott Brown in his loss to Elizabeth Warren.
At the time, the SEIU unions and the Massachusetts Teachers Association had collectively poured hundreds of thousands of dollars into the PAC, which sat dormant for years and had roughly $75,000 to start July. It has yet to report any new donations.
Similarly, the Toughest Fights PAC, which is running the TV ad backing Mermell and is coordinating with the labor-backed group, has not yet disclosed its donors beyond saying they include “women and progressive” Democrats.
They’re far from alone in the race. A group known as Unite to Win has formed to back Alan Khazei, a Brookline Democrat and cofounder of City Year, who Thursday touted the endorsement of former Obama defense secretary Leon Panetta.
And a super PAC funded, in part, by Auchincloss’s parents and a national veterans group has so far dropped more than $420,000 on television ads and mailers to bolster the US Marine veteran’s candidacy.
It’s by far the most outside spending by any single group thus far in the race, and adds to the deluge of television advertising flooding into Fourth District voters’ homes.
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As least five candidates — Auchincloss; Grossman; Leckey, a former Wall Street regulator; tech entrepreneur Christopher Zannetos; and Khazei — have collectively launched more than $2.2 million in television ads in recent weeks, expanding their reach to voters as the novel coronavirus pandemic limits in-person campaigning.
Ben Sigel, a Brookline attorney, and Natalia Linos, a social epidemiologist at Harvard University, are also vying in the primary.
Matt Stout can be reached at matt.stout@globe.com. Follow him on Twitter @mattpstout.