With U.S. Rep. Ayanna Pressley’s announcement earlier this week that she’s not running for U.S. Senate and another candidate, Alex Rikleen, hovering in the single digits , the Democratic primary looks likely to be a two-person race between incumbent Ed Markey and challenger U.S. Rep. Seth Moulton. That raises an obvious question: who gains with Pressley on the sidelines — the guy trying to keep his job, or the guy trying for a promotion?

Ask a Massachusetts political observer, and you’re likely to hear that Pressley’s decision not to run was cause for celebration at Markey HQ. The argument goes something like this: Markey and Pressley are both progressive Democrats, and would have been vying for largely the same bloc of voters if she’d entered the race. Consequently, Moulton could have eked out a victory if he’d attracted enough moderate Democrats and Democrat-leaning independents.

When Pressley took herself out of the running, said UMass Boston political scientist Erin O’Brien, “Markey’s camp was mighty excited. Had Ayanna Pressley got in the race … with his progressive bona fides, he would have had a challenge there. So I think the Markey camp was incredibly happy.

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“I think there was an equal and opposite reaction at Seth Moulton’s camp,” O’Brien added. “His campaign would have loved to have progressives in the primary divide the vote, and then he could get moderate Democrats and the occasional Massachusetts independent who might pull a ballot in the Democratic primary.”

Pollster David Paleologos of Suffolk University agrees.

“It’s a win for Ed Markey and it’s a setback for Congressman Moulton,” Paleologos said of Pressley’s decision not to run. “What the polling data told us, when we put Ayanna Pressley on the … test ballot was that Democratic voters, progressive voters, are willing to consider a younger alternative provided that the progressive bona fides are being met. Meaning they were willing to take a flier with Ayanna Pressley against Ed Markey, but not willing to take a flier, at least at this point, with Moulton against Markey, because Moulton’s not seen as progressive enough for Democratic primary voters.”

Here’s how that dynamic played out in the most recent poll Paleologos conducted for The Boston Globe. In a two-way matchup, Markey led Moulton by a relatively comfortable margin, 45% to 22%. But with Pressley in the hypothetical mix, she actually led Markey 35-34, with Moulton at 16%. The margin of error among likely Democratic primary voters was 6.5%.

It’s worth noting, though, that the results were a bit different in a UMass poll for WCVB-TV released in early November. That survey, too, showed Markey leading Moulton without Pressley on the test ballot, 51% to 28%, with Rikleen at 6%.

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With Pressley on the test ballot, Markey still led Moulton, but by a much slimmer margin, 35-25; Pressley garnered 21% of the vote and Rikleen finished with 5%. The margin of error here for likely Democratic voters was 6.1%.

This is where a bit of ambiguity starts to creep in. Moulton’s case against Markey is quite literally that he’s too old, and that it’s time for a “new generation of leadership” in the Democratic Party. As Moulton put it in his campaign kickoff video, “I don’t think someone who’s been in Congress for half a century is the right person to meet this moment and win the future.”

In a contest with Markey, Moulton and Pressley, voters who agree with this diagnosis would have had two prominent options to consider instead. In a contest with just Markey and Moulton, they have one. Rikleen’s polling numbers suggest that, for now, he’s unlikely to be a major factor in the race.

To put it just a bit differently, if Pressley could have helped Moulton by splitting the progressive vote with Markey, she could also have helped Markey by splitting the generational-change vote with Moulton. In fact, as the first Black woman elected to Congress in Massachusetts history, she might have been an even more obvious choice than Moulton for voters who consider change a top priority.

Now, though, Moulton gets to run on the binary equation he started the campaign with: he’s young, Markey’s old, and it’s time for a new generation to take the reins and lead the fight against President Donald Trump and his administration.

That’s the argument made by Moulton strategist Doug Rubin, who’s convinced that dissatisfaction with the political status quo will be the dynamic that ultimately drives the Democratic Senate contest. He cites Democratic wins in the New Jersey and Virginia governor’s races, Zohran Mamdani’s mayoral win in New York City, and incumbent mayors losing in Somerville and Everett to make his case.

“Voters are frustrated with what’s going on right now, and they feel like whoever the leadership’s been to date has gotten us to a point where we’ve got a second term of Trump and a totally MAGA Congress that is attacking working families and destroying our democracy,” Rubin said. “And I just think that they are going to be looking for new leaders to get us out of this mess, and not elect the same old same old.

“I just think that is kind of the dominant narrative in this race,” Rubin added. “I think a one on one race with us and Markey is the best chance for Moulton to win this thing.”

Winelia Rivera, who was Pressley’s chief strategist when she unseated then-U.S. Rep. Mike Capuano in the 2018 Democratic Primary, also questions the assumption that Pressley’s decision elicited cheers from Markey’s camp and dismay from Moulton’s. While she thinks Markey was probably “elated” by Pressley’s choice, Rivera says, the mood was likely pretty upbeat at Moulton HQ as well.

If she were a Moulton loyalist, Rivera said, “I would have been excited, because it makes it a one on one race with Markey as opposed to diluting the votes across the state … It gives him a different shot at a path to victory. I think it increases his possibilities.”

Rivera also notes that Moulton and Markey are facing off at a time when communities like Lawrence and Fall River are experiencing a rightward shift that’s remaking Massachusetts’ electoral map.

“Some of the reliable votes that the Democratic Party has relied on statewide just aren’t as reliable, especially at the working-class level, beyond Greater Boston,” Rivera said. “And when you look at where Markey either had to really compete or won by a very close margin” — in his 2020 primary contest against Joe Kennedy III — “[it was] in gateway cities across the state. So I do think that, from an electoral perspective, the conditions for [a Moulton win] are there.”

There is, Rivera believes, one big caveat: Moulton’s ability to run an effective statewide campaign remains to be seen. Here’s another: Massachusetts Democrats may be unswayed by Moulton’s “generational change” argument and decide en masse to reward Markey for decades of progressivism in Congress. But with the primary election nine months away, and the contours of the campaign just now coming into focus, it’s a bit early to assume that’s the only way the race could play out.