When U.S. Rep. Chrissy Houlahan decided she wouldn’t run for Pennsylvania’s U.S. Senate seat, moderate Democrats lost one of their big hopes for next year’s critical election.

But they haven’t freaked out. That’s because in Democratic political circles, there’s widespread belief that U.S. Rep. Conor Lamb, a Western Pennsylvania Democrat with centrist credentials, will eventually join the fray.

Messages to two centrist party insiders yielded almost identical responses: No one is worried about Lamb. The expectation is he’ll be in the race.

Lamb, who has openly discussed a potential campaign, is one of the last major Democrats yet to announce his intentions after Houlahan last week and U.S. Rep. Madeleine Dean this week each said they won’t run. Their decisions clarified the Democratic primary field — and put even more focus on Lamb — as the party eyes a contest that will determine who runs for an open seat in one of the country’s most competitive races, potentially deciding control of the Senate. (Incumbent Republican Sen. Pat Toomey isn’t seeking reelection.)

The GOP field has a few established contenders, but there’s a sense among Republicans that their primary may be more fluid, with more potential candidates in the offing.

Here’s how the Democratic developments, and a potential Lamb campaign, could affect the shape of the primary contest:

Conor Lamb and the center-left lane

To some Democrats, Lamb fits the Joe Biden-Bob Casey mold that has worked in Pennsylvania: Maybe not as bold on policy or exciting as more liberal candidates, but a pragmatic choice who can appeal to a wide swath of voters in a closely divided state.

He has won three congressional races in districts that either leaned heavily or somewhat conservative in Southwestern Pennsylvania. Biden campaigned for him, and in turn Lamb was a key surrogate for Biden. He’s a former Marine and military prosecutor who drew national attention with a 2018 special election win that foreshadowed the Democratic wave to come.

» READ MORE: The Divided States of Pennsylvania: How one state embodies America's political discord

With Houlahan out of the race, Lamb would have a clear path as the more moderate choice for Democrats who lean that way — or who just believe candidates like him are needed to win statewide. But some of his centrist votes could prove problematic in a Democratic primary, especially as the party’s progressive wing exerts more influence.

For example, he has opposed a ban on assault-style weapons (though in April shifted his stance to support bans on future assault weapon sales), and in December voted against a bill to legalize marijuana. (He said he supports decriminalization but that the specific bill went too far.)

Val Arkoosh and the Philly suburbs

Perhaps the biggest beneficiary of Dean and Houlahan taking a pass is Montgomery County Commissioner Val Arkoosh. There could have been up to three well-established Democratic women from the Philadelphia suburbs — Houlahan is from Chester County, and Dean is also from Montgomery.

Now there’s one.

That could be a powerful thing: Suburban women have been a key piece of the Democratic coalition since 2018, and Montgomery is the state’s third largest county.

While Philadelphia and Allegheny Counties are (by far) the biggest pieces of the Democratic pie, Philly’s collar counties combined for 24% of the party’s 2020 presidential primary vote — more than either of the state’s big cities.

Arkoosh will still face competition in those places, but she has a much clearer path.

She’s also now the only well-known choice for influential women’s groups who hope to see Pennsylvania elect a woman to the Senate for the first time. That could also be a major factor, as it was in 2016 when EMILY’s List, a key Democratic group that supports women, played a powerful role in propelling Katie McGinty to the party’s Senate nomination.

John Fetterman and Western Pennsylvania competition

After running a long-shot campaign in 2016, John Fetterman is widely seen as the early Democratic front-runner. As lieutenant governor, he has won statewide, developed a fervent following among some progressives, and posted big early fund-raising numbers for a race that could break spending records.

Houlahan was the one Democrat who might have rivaled his financial might.

From a geographic perspective, while Arkoosh’s path is clearer, Fetterman’s may become more complicated. He and Lamb are both from Allegheny County, and could potentially split voters in and around Pittsburgh.

And while Fetterman is often seen as a progressive candidate who can also reach white, working-class voters, Lamb can point to his 2018 victory in a district that heavily favored former President Donald Trump.

Fetterman has preemptively tried to take hold of the “electability” debate by releasing polling from the progressive group Data for Progress to argue he would be the strongest general election candidate.

Of course, that comes before any serious campaigning has begun.

Malcolm Kenyatta and Philadelphia

State Rep. Malcolm Kenyatta, of Philadelphia, was seemingly less directly affected by this month’s moves. He’s widely seen as a charismatic progressive who could battle Fetterman for support on the left. He was already the only well-established Philadelphian and person of color in the contest.

None of the recent maneuvers changed those factors.

Still, having fewer candidates from the Southeast could give him more room to run in Pennsylvania’s most populous region. Philadelphia and the suburbs accounted for 43% of Democrats’ 2020 presidential primary votes.

And while he represents the city, many suburban Democrats might be attracted to the idea of electing a Black, gay man who would break several barriers if he joined the Senate.

One potential hurdle: State Sen. Sharif Street, also of Philadelphia, has formed an exploratory committee, and if he runs his profile would overlap with Kenyatta’s in some ways.

There are a number of less well-known Democrats running, and still the potential for a surprise entry. For now, though, the major players appear set.