Democrat Jeff C. McKay won the chairman’s seat of the Fairfax County Board of Supervisors on Tuesday, while veteran Republican Supervisor Pat Herrity narrowly hung onto what will be the sole remaining GOP seat on the board.

Democrats flipped the seat held by outgoing Supervisor John C. Cook (R-Braddock), and will hold a 9-1 majority when the new members take office in January.

The arrival of several more liberal voices to a historically moderate county government puts Virginia’s most populous jurisdiction on a path to exert greater influence on Democratic issues in the state, made easier by the party’s success Tuesday in winning control of the General Assembly.

McKay (D-Lee), 44, beat Republican Joseph Galdo to replace longtime Chairman Sharon Bulova (D), who is retiring next year after 30 years on the board.

Herrity, an 11-year incumbent, defeated Democrat Linda Sperling in the Springfield district by about 600 votes out of more than 40,000 cast.

McKay has promised to “hit the accelerator” on affordable housing, climate change and social equity, themes that also defined the nine other board races. His opponent built his campaign around a desire to return to what he described as the county’s tranquil suburban roots, calling for stepped-up immigration enforcement and vowing to “Make Fairfax Safe Again.”

“Fairfax County is more Democratic than ever,” McKay said Tuesday night. “What is clear is that more work needs to be done to protect the environment, double down on affordable housing, and to respond to some of these other critical issues where the state government and federal government aren’t doing what they need to do at all.”

Democrat James R. Walkinshaw beat Republican Jason Remer in Cook’s Braddock district. The other Democratic candidates who defeated Republican opponents Tuesday were incumbent Supervisors Kathy L. Smith (Sully), John W. Foust (Dranesville) and Penelope A. Gross (Mason); and newcomer Dalia Palchik (Providence), a four-year member of the Fairfax County Board of Education.

Supervisor Daniel G. Storck (Mount Vernon) and Democrats Rodney Lusk (Lee) and Walter Alcorn (Hunter Mill) were unopposed.

McKay won the top elected post in Fairfax after 11 years on the board as the Lee district supervisor. In that seat, he was a steady and measured voice for liberal policies in the county, even as his critics said he was too close to developers.

He co-sponsored a 2017 “One Fairfax” policy that aims to use social and racial equity as a prism for county spending and planning and has pushed for the county to do more to help undocumented immigrants facing deportation.

In his district, McKay helped drive plans to redevelop the Richmond Highway corridor and spearheaded the renovation of a once crime-ridden Springfield mall.

Along the way, McKay became Bulova’s protege, backing her cautious approach in dealing with an economic slump that made it harder to meet the demands of an increasing population of immigrants, elderly residents and low-income families.

Bulova appointed McKay to lead the board’s important legislative and budget committees — putting him in position to make a case to voters that he was the most qualified to take her spot when she decided to step down.

However, that plan was jeopardized during the Democratic primary election, when one of McKay’s opponents — developer Timothy M. Chapman — accused the supervisor of benefiting from a quid pro quo relationship with a developer friend. Chapman alleged that McKay got a bargain price on the home he and his wife bought in 2017, in exchange for pushing action by the board that the developer wanted.

A formal complaint filed by Chapman triggered a yet-to-be concluded state police investigation, which McKay has said was unwarranted, calling the allegations unfounded and “borderline libelous.”

Galdo, 73, said the allegations showed that McKay and other county supervisors were too cozy with developers. But several voters said the allegations didn’t concern them.

“I don’t think Republicans have any place talking about quid pro quos,” said Glory Fox Dierker, 73, referring to the House impeachment inquiry into whether President Trump withheld foreign aid to Ukraine for personal political gain.

Some Galdo voters said they agreed with the Republican’s call to abolish the One Fairfax policy McKay championed, criticizing it as a form of social engineering that local governments should not embrace.

“It sounds beautiful, and I believe in social equity,” said Goldie Fossa, 35, after voting at West Springfield High School. “But the more government tries to intervene to make everyone equal, the more it ends up hurting everyone.”

Galdo said in an interview Tuesday night that he hopes to work with McKay on some of the other issues he raised, such as controlling high-density development in areas where surrounding roads and other infrastructure aren’t equipped to handle more people.

“They have to have a more realistic outlook for growth in the county,” he said.

Herrity, 59, fended off Democratic efforts to take his seat. A voice of fiscal restraint on the board, Herrity made a case for partisan balance to voters in his changing district while highlighting his work to increase services for the elderly and curb escalating pension costs.

Democrats tried to turn out the vote for Sperling, 42, in an area of Fairfax that overlaps with the House district where longtime Del. Timothy D. Hugo (R-Fairfax) was ousted by Democrat Dan Helmer. Sperling campaigned to add more permanent classrooms, build more affordable housing in some areas slated for single-family homes, and increase access to public transportation.

Peggy Probst, 76, said she voted for Herrity because “he seems to be for the people.”

“We’re familiar with him,” she said, adding that she frequently sees Herrity at community events and likes that his father — former county board chairman John F. “Jack” Herrity (R) — was also involved in local politics.

Burke resident Mel Jaranson, 76, also recognized Herrity’s long record of service. But he cast his vote for Sperling.

“I just don’t much care for the Republican agenda,” said Jaranson, a retiree. “It’s good to see Virginia turning blue.”

In Braddock, Walksinshaw — chief of staff to Rep. Gerald E. Connolly (D-Va.) — pledged to bring more density to the Fairfax Corner area and stop traffic flowing onto residential streets from Braddock Road.

Remer, a mechanical engineer who focuses on renewable energy, said he would champion a program that steers criminal suspects with mental illness or addiction to treatment instead of jail.

Alan Olmos, 32, said he voted for Walkinshaw and McKay because he wants a local government that supports immigrants. Referring to Trump, he said: “I voted the way I voted based on who is in the government right this second.”

Rachel Chason contributed to this report.