Fairfax County’s Board of Supervisors on Tuesday took a decisive step toward making itself the sole provider of water service in some areas and setting rates for all its residents, including those who get their water from nearby municipalities such as the City of Falls Church.

But a Republican supervisor suggested the Board’s Democratic majority was getting ahead of itself on a complicated issue simply to score points before next Tuesday’s election.

The board’s 9-to-1 vote sets up a public hearing Dec. 6 to formally consider enacting an ordinance allowing the board to establish service areas for Fairfax Water, the county-controlled utility that supplies most water in the county. The proposed ordinance would also allow the county to set maximum water rates and fees for residents, even if their water comes from a municipal utility, unless those municipal utilities justify their higher rates.

But officials in neighboring jurisdictions say they are troubled by the board’s unexpected show of muscle. Falls Church officials have said the county wants a monopoly in a long-running battle over water service. The victor could lay claim to millions of dollars by supplying water to new development in Tysons Corner and Merrifield.

“It’s a little heavy-handed,” said Vienna Mayor M. Jane Seeman, adding that the town’s attorney believes the move would run counter to state law.

Although the action sets Northern Virginia’s largest jurisdiction on a potential collision course with its smaller neighbors, board Chairman Sharon Bulova (D) and other supporters argued that the move was necessary to prevent municipal water utilities from imposing higher rates on county residents who have little power to fight back.

Supervisor Linda Q. Smyth (D-Providence) read a letter from some constituents saying their neighborhood survey found that a family living in the county and receiving Vienna water pays approximately $100 more than neighbors who receive Fairfax water.

“As for Falls Church, my constituents are up in arms, and they have been there for many, many years,” Smyth said.

The county’s Consumer Protection Commission this year said that under Falls Church’s new water rates, a county resident who receives city water would pay about 60 percent more than a neighbor who receives Fairfax Water. The commission also also found that Falls Church effectively passed off the costs of system expansion to existing customers — an assertion the city has disputed. Barbara J. Gordon, a Falls Church spokeswoman, said the city has supplied some county residents with water since the 1930s, and that city and county residents have paid the same rate for Falls Church water since the 1970s.

“I don’t view this as our trying to trample over other jurisdictions,” Supervisor Gerald W. Hyland (D-Mount Vernon) said. “But I view this as a step to say, ‘Enough is enough.’ ”

But Supervisor John C. Cook (R-Braddock) criticized the county’s “Standard Oil approach” and argued for more time to work out an amiable political solution with its neighbors.

“Otherwise, we’re going to have a lawsuit,” Cook said.

Supervisor Pat S. Herrity (R-Springfield), who cast the lone vote against the measure, said the board’s Democratic majority hurried the vote without considering all the possible consequences because of election-year politics. He said the board appeared to be trying to bolster the reelection campaign of Supervisor John W. Foust (D-Dranesville), whose district had been in Republican hands for 12 years before his 2007 victory. Foust is facing a spirited challenge this year from Dennis Husch, a Republican who served on Herndon’s Town Council. Their district is also directly affected by the long-running legal battle between Falls Church and Fairfax Water and the remaking of Tysons Corner.

“It’s rushing it so that we can say we did something before Election Day,” Herrity said in an interview before the meeting.

But Foust said during the meeting that the matter was simply about fairness.

“If we allow Falls Church to continue to serve Tysons Corner, then we know who pays. And that’s the residents of Fairfax County,” Foust said. “This is the best shot we have at bringing equity to Fairfax County residents.”

The dispute began over Falls Church’s treatment of about 100,000 customers who live in Fairfax County but receive city water. In years past, the city billed those customers above cost and transferred the surplus to its general fund. From 1981 to 2008, Falls Church directed more than $58 million in surplus revenue to the city treasury — a practice city officials say is common among municipal utilities.

But the course set by Fairfax County in recent weeks could affect all the municipalities that provide water to county residents.

City of Fairfax Mayor Robert F. Lederer said he was surprised by Fairfax County’s move, especially because the city charges its own residents about 10 percent more than county residents. But for now, he is taking a wait-and-see approach.

“We’ve been providing city and county residents with water for 50-plus years at a very competitive rate, with a high quality of water and customer service, and we expect to continue doing that,” Lederer said.

Steven Briglia, Vienna’s town attorney, said the county appears to be heading down a path that conflicts with state law, which allows municipalities such as Vienna to charge a reasonable rate, even with justifiable surcharges for nonresidents. But Briglia also said Vienna would prefer to hash things out with the county rather than battling in court.

“Every single person in the town of Vienna is a citizen of Fairfax County,” Briglia said. “So the last thing we want to do is sue ourselves.”

This post had been updated.