SEPTA is dominated by Republicans.

That’s been an ironclad reality since the agency was founded in 1963, but it’s likely to be turned upside down, thanks to the results of Tuesday’s elections.

Democrats’ newly won control of boards of commissioners in Chester, Delaware, and Bucks Counties means the Republicans representing those counties on SEPTA’s board will likely be replaced by members of the victorious party when current terms expire.

The three boards that went Democratic will remain that way until the next election in 2023. In 2022, Chester County’s commissioners will be the first of the newly flipped bodies to make SEPTA board appointments. Adding two Democrats would give that party a three-vote majority on SEPTA’s 15-member board, which oversees an agency with a $2 billion combined operating and capital budget.

It’s too soon to say what the change could mean for SEPTA’s 430,000 daily riders, said Marian Moskowitz, one of two Democrats elected Tuesday as a Chester County commissioner, giving her party the majority there for the first time.

“You don’t want to assume anything, and now it’s just one night that I won,” she said. “It’s going to take me a while to understand the roles and how they all work.”

The transit agency’s governing board is made up of two representatives from each of the five counties it serves; four appointees from the legislature, with both parties in both houses represented; and one member appointed by the governor. Chester, Delaware, and Bucks Counties’ six board members are Republican. Along with the legislative appointees, that gives the current board an 8-7 tilt in favor of the GOP.

It is unclear whether a shift in parties will change one of the fundamental dynamics of SEPTA’s board: the tension between urban and suburban interests. Philadelphia, strongly Democratic, has more public transit riders than the surrounding four counties combined, census data show, but the city has only two board seats.

City government has been increasingly engaged with SEPTA in recent years. Efforts to reduce congestion and increase transportation access for people in the city’s poorer neighborhoods have required greater cooperation between city government and the transit agency, and the two have not always seen eye to eye. The city has been a driving force pushing for a revamped bus service, which has been bleeding riders and is due for a shakeup in the next two years. Most recently, the city has pushed for SEPTA to eliminate transfer fees for riders switching from one mode of travel.

In Chester County, Moskowitz, a businesswoman, backs a rail extension to Phoenixville, something SEPTA has discussed but has no tangible plans to build. SEPTA has prioritized a program of Regional Rail station upgrades and replacements throughout its rail network and has described a light rail extension to King of Prussia as a priority.

Helen Gym, a City Council member who has been critical of SEPTA, is hopeful a leadership change on the board could usher in a new vision to reverse the trend of declining ridership since 2014.

“Public transit has an enormous role to play getting people to jobs, education, and schools,” Gym said. “I really do hope that a Democratic SEPTA leadership will be more responsive to that than what we’ve seen in the past.”

SEPTA’s current management emphasizes that good transportation boosts economic development and job access, and supporting Philadelphia ultimately helps the entire region and state. Gym said she hoped new Democrat board members would embrace that ideal.

Democratic control of the board could jeopardize chairman Pasquale M. “Pat” Deon’s 20-year dominance at the transit agency. Deon, a Bucks County businessman and an influential Republican statewide, is described by members of both parties as a unique asset to SEPTA as an advocate for transit in Southeastern Pennsylvania who has clout in Harrisburg, where Republicans have a legislative majority.

In 2022, when Chester County is due to appoint new board members, Pennsylvania will face the possibility of losing $400 million in transportation funding, part of which goes to SEPTA. Should Republicans continue to control the legislature then, Deon is expected to play a significant role in lobbying for transportation funding.

Deon was not available for an interview Thursday, a SEPTA spokesperson said. He has said he may retire in 2024 at the end of his current five-year board term. The board selects a new chairman annually.

Transportation’s effect on the environment was an important element of the winning Democrats’ platform in Chester County, Moskowitz said.

Conservation Voters of Pennsylvania, an environmental advocacy group, spent $250,000 on mailings, canvassing, and ads to support the Democratic candidates in Chester County. The greatest concern among environmentally minded voters was the Mariner East pipeline, which carries Marcellus Shale natural gas liquid to Marcus Hook, and plans to build another pipeline, said the organization’s executive director, Josh McNeil. But the group also focused on the ways SEPTA could be more environmentally friendly.

SEPTA recently committed to obtaining almost 20% of its energy through solar power and has plans to run 35 electric buses in a fleet of about 1,500.

The group focused on Chester County because it seemed more likely Republicans would perform more strongly there than in Delaware County, he said, and because its residents seemed receptive to an environmentally friendly campaign.

“There’s a connection to nature," McNeil said, "that is very real and deep in that county.”

This story has been updated to clarify the city’s role in revamping SEPTA’s bus network, and to state Republicans were expected to be more competitive in Chester County.