The long-pursued vision of Washington and its sprawling suburbs tied together by Metrorail took a leap forward Tuesday as Loudoun County committed to support Metro’s Silver Line extension to Dulles International Airport and into eastern Loudoun.

After months of heated debate and tense negotiations, the 5 to 4 vote by the Board of Supervisors appeared to end the latest, and perhaps the last, round of uncertainty over the fate of the Silver Line.

Estimated to cost $6 billion, the project has survived not only the recession but also a succession of political battles over its price tag— from the debate over putting the extension above or below ground to the recent fight over a labor-friendly contracting practice.

After the labor issue was resolved last month, Gov. Robert F. McDonnell (R) withdrew his threat to withhold $150 million earmarked for the rail extension. That left the board as the last remaining hurdle, and in recent days, McDonnell had urged the board not to stand in the way.

With his decision to support the Silver Line after strongly opposing it, Supervisor Kenneth D. Reid (R-Leesburg) delivered the fifth and deciding vote for a project that supporters predict will have a lasting impact.

“This is a huge investment in Loudoun’s future,” said Tony Howard, president of the Loudoun Chamber of Commerce, which represents 1,200 businesses. “It’s important not just for our economy in the next quarter or the next year, but for a payoff that will be generational. . . . The chamber is absolutely elated.”

The Silver Line’s first leg, expected to be running by August 2013, will pass through Tysons Corner to Wiehle Avenue in Reston. When the entire line — which is being built by the Metropolitan Washington Airports Authority — is completed, it will span 23 miles of rail between East Falls Church and Ashburn.

For the first time, some of the region’s more distant Northern Virginia suburbs will be linked by rail to the rest of the metropolitan area. A traveler who would rather not pay upward of $60 to take a taxi from Dulles to the District will have a new option. So will someone who is able to land a job at the airport but won’t have a car to get there every day.

If Loudoun had backed out of its commitment to contribute to construction of the second phase of the Silver Line — a share now estimated at $270 million — the extension to Dulles and into Loudoun would have been in jeopardy, and with it a major rationale for the Silver Line.

For a once-rural community that has become one of the fastest-growing counties in the nation, the addition of rail marks a significant step for Loudoun — one that has been championed by many county residents and business leaders and especially by young adults, who have said that access to the District is a key factor for 20- and 30-something professionals looking to settle in the suburbs.

But the debate and the narrow margin of Tuesday’s vote revealed a lack of unity among the all-Republican board as the supervisors and other leaders wrestle with Loudoun’s evolving identity. The supervisors were swept into office in November on platforms emphasizing fiscal responsibility and economic development — two ideals that proved challenging to balance when it came to Silver Line.

The Loudoun board’s recent discussions focused on establishing a financing plan for the county’s share of the construction costs, and the board voted Tuesday to pursue a proposal that would fund the project through the establishment of a special tax district. The district encompasses commercial property and undeveloped land immediately surrounding the Metro station locations and excludes nearly all existing residential property.

In addition to the construction costs, Loudoun will contribute to the Washington Metropolitan Area Transit Authority’s annual operating costs beginning in 2019, with an initial payment of $17 million, according to county staff.

The funding plan proved pivotal for Reid, who joined four board members who had consistently advocated in favor of the rail project. The proposed tax district met Reid’s goals to pay for Silver Line costs without relying on the county’s general fund or a countywide tax, he said.

“This essentially is something that has allowed me to lean to the direction of supporting the project,” Reid said.

Tuesday’s vote was applauded by elected officials and project partners, including McDonnell.

“The governor is pleased by today’s vote to extend Metrorail to Dulles Airport,” said McDonnell spokesman Tucker Martin. “This is a critically important project to both reduce congestion as well as spur economic growth and job creation in Northern Virginia.”

Fairfax Board of Supervisors Chairman Sharon S. Bulova (D) also said she was pleased that Loudoun opted to remain committed to the rail extension.

“I look forward to continuing to work to maximize the economic development potential that rail brings,” Bulova said in a statement.

State Sen. Mark R. Herring (D-Loudoun) said the board’s vote was “an investment in our future and will help ensure our region’s continued economic growth and prosperity.”

Metro officials also praised the vote. “Today’s vote enables the long planned extension of the Metrorail system along the Dulles Corridor, with the companion benefits, to now proceed,” Metro Board of Directors Chairman Catherine Hudgins said in a prepared statement.

But Audrey Jackson, state director for Americans for Prosperity, a powerful conservative group that paid for thousands of robo-calls lobbying against the Silver Line extension, warned that the group’s Virginia activists would “remember the five members of this Board of Supervisors who supported this project, a project even the federal government didn’t see the need for.”

The group will “watch closely how the board plans to pay for it,” Jackson said. “Taxpayers are not going to be pleased with higher property taxes, more congestion in the region and higher tolls.”

The four supervisors who voted against the project raised concerns about the potential for additional costs. They questioned whether the project would relieve traffic congestion and expressed skepticism about the risk of an indefinite partnership with the MWAA and WMATA.

Vice Chairman Janet S. Clarke (R-Blue Ridge) and Supervisor Geary M. Higgins (R-Catoctin) said the question wasn’t about whether to support rail, but whether the deal in question was the right one for Loudoun.

The proposed agreement “really turns my stomach,” Clarke said. “There are so many aspects of it that are not going to help the county.”

But Supervisor Ralph M. Buona (R-Ashburn), who was among the five who voted in favor, argued that the choice was really between the best interests of the county and individual political gain.

“You have to take a holistic view and say, ‘What is best for the people?’ You can say, ‘I was a visionary,’ or ‘I was one of those people who said we should never build rail in Loudoun County,’ ” Buona said. “This is the vote of a decade, maybe a generation, maybe a century.”

Officials with the MWAA said they would immediately begin work to identify and hire a firm to build the second phase of the rail line.

Pat Nowakowski, executive director of the Dulles Metrorail Project, said the process would take about nine months. He expects some work to begin next year, with the bulk of the heavy construction starting in 2014.

“We’re ready to go,” Nowakowski said. “We’re ready to move this project forward.”

Anita Kumar and Lori Aratani contributed to this report.