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Representative Joseph P. Kennedy III is running. Just about everyone else may be, too.

Kennedy’s plan to challenge Senator Edward J. Markey has galvanized elected officials, activists, and other politicos in the 38-year-old’s winding Fourth Congressional District, where nearly a dozen Democrats have said they’re weighing a campaign.

The once-unexpected opportunity to seize an open seat in Washington, combined with the party’s deep bench in Massachusetts, has created a potential free-for-all congressional primary to replace Kennedy, who’s represented its 34 cities and towns since 2013.

A shadow race has already been churning for weeks, with potential candidates assembling staff and calling supporters. But with speculation about a Kennedy run transforming into expectation — the four-term congressman is planning to launch his Senate campaign Saturday — the whispers about a growing field are becoming far louder.

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“It’s almost political malpractice if you don’t take a look at it,” said state Representative Patricia A. Haddad, the House’s third-ranking Democrat and a Somerset resident who’s weighing a potential campaign. “You have to do it.”

Before Kennedy’s pivot to the Senate, just one challenger — Ihssane Leckey, a self-described democratic socialist from Brookline — had entered the race.

Democrats expect the field to grow significantly, if not quickly. Looming: the start of a new fund-raising quarter on Oct. 1, which might prompt some candidates to wait to declare until after that date.

But many also indicated they won’t wait long to make a decision.

Among the more established candidates considering a run is state Treasurer Deborah B. Goldberg, a 65-year-old Brookline resident who already took a step toward a campaign by filing a statement of candidacy with the FEC. But an adviser says she’s genuinely torn about potentially giving up her current seat, setting the odds of her running at “50/50.”

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Others are sending stronger signals. Jesse Mermell, a former Brookline selectwoman, stepped down as president of the Alliance for Business Leadership this week to prepare a campaign, according to a source close to her.

A communications director to former Governor Deval Patrick, she’s enlisted David Guarino, of the firm Melwood Global, and Bob LaRocca as consultants, as well as Danielle McCourt to handle her campaign’s finances.

Becky Walker Grossman, a Newton city councilor, said she intends to make a decision “very soon” on whether to launch a campaign. A former prosecutor, Grossman is the daughter-in-law to Steve Grossman, a former state treasurer and former national Democratic Party chairman who lives in Newton.

“I’m definitely thinking very seriously about this,” she said.

The Fourth District is stitched together by various political hubs, including blue-collar cities to the south in Fall River and Taunton, and liberal suburbs to the north in Newton and Brookline.

It’s there where several others are also weighing their chances. State Representative Tommy Vitolo, a 40-year-old Brookline Democrat elected in November 2018, has said he’s considering a campaign. (He did not return a call for comment Friday.) So is Grossman’s colleague on the Newton city council, Jake Auchincloss, a former Marine who served in Afghanistan.

“There are people encouraging me. I’m taking a really serious look at it,” he said Friday.

Dave Cavell, a former speechwriter to President Obama and who grew up in Brookline, said he’ll make a decision in the coming weeks on whether he, too, jumps in. The 35-year-old is currently a senior adviser to Attorney General Maura Healey.

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But Democrats are also watching whether serious candidates could emerge from the southern end of the district, offering a potential geographical advantage in what could be a divided fight for support

Haddad, a Somerset resident, pointed to the surfeit of would-be candidates from the Boston suburbs, as if “this is a seat from there.”

“I just want to make sure that the person, or the people who are considering, look at the uniqueness of what we have down here,” said Haddad. “I’m not going to dismiss [running] out of hand, but I’m not ready to jump in with both feet.”

State Senator Paul Feeney, a Foxborough Democrat and former labor leader, said he intends to take the coming days before making a decision.

“It’s a defining time in our country,” he said of the draw of the seat. “It’s not something you want to rush into it.”

The dynamic of a suddenly open congressional seat played out just last year in the Third District, where 10 Democrats ultimately piled onto the ballot in bids to succeed Niki Tsongas. Representative Lori Trahan ultimately prevailed by a mere 145 votes in a recount before beating a Republican in November.

Less clear in the Fourth, however, is who from the GOP could emerge to run in the Democratic-leaning district. State Representative Shawn Dooley, of Norfolk, said he wouldn’t comment “one way or another” on a potential run until after the big Saturday announcement — Kennedy’s.

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Reach Matt Stout at matt.stout@globe.com.