At a standing-room-only meeting in Leesburg to address the extension of Metro’s planned Silver Line into Loudoun County, opponents of the project had the first say. But it became clear that most of the more than 100 speakers had come to urge the Board of Supervisors to support bringing Metro to Loudoun.

The opening lineup of about a dozen opponents reminded officials of campaign promises of fiscal responsibility and lower taxes. “It’s not the project I object to, Mr. Chairman,” Del. Bob Marshall (R-Prince William) said, addressing Board of Supervisors Chairman Scott K. York (R-At Large.) “It’s the cost of the project.”

Marshall questioned whether the county should trust the Metropolitan Washington Airports Authority’s management of the project. “You are going to have no control over these costs,” he said. “I urge a no vote.”

The first phase of the project, which runs through Tysons Corner to Wiehle Avenue, is expected to be finished in August 2013. But plans for the second phase, which is to run to Dulles International Airport and into Loudoun, have been slowed by concerns about the cost of construction and objections from Republicans to a planned labor agreement.

At the meeting Monday night, opponents were outnumbered by supporters, many of whom wore green, intended to signify their support of the project. A steady stream of residents and business leaders voiced fervent support, reminding Loudoun’s all-Republican board of its pledge to help attract new businesses and expand the county’s commercial tax base.

Grafton DeButts, a founding member of the Loudoun Young Professionals initiative, said the extension was critical to retaining and recruiting young talent.

“We don’t have that urban center that drives young professionals to Loudoun County, like Arlington or Reston, Rosslyn or the District,” he said. “We have great jobs here, but it’s about where they go for community.”

Victoria Rawlings, a young professional who recently moved to Loudoun from California, echoed DeButts’s sentiments. She said she moved to Sterling rather than the District, but noted that it was a difficult choice.

“Every day, young professionals are coming in and making a very different decision,” she said. “Please don’t miss out on the people who are going to continue to grow this community.”

Before the meeting started, supporters and opponents of the project rallied on the government center grounds and on opposite sides of Loudoun Street, drawing honks and waves from passing motorists.

As the July 4 deadline for the Loudoun board’s deciding vote on the project approaches, advocates and opponents of the Silver Line have heightened efforts to be heard. Last week, the conservative group Americans for Prosperity made thousands of robo-calls urging Loudoun residents to oppose the extension, saying county taxpayers should not have to foot the bill for the project.

Meanwhile, local business leaders and labor organizations have increasingly voiced support for the Silver Line in recent weeks, running ads in local papers and urging Loudoun supervisors to consider the potential economic benefits of bringing Metro into the county.

Tony Howard, president of the Loudoun County Chamber of Commerce, addressed the board Monday night on behalf of the chamber’s 1,200 member businesses.

“The long-term benefits of the Dulles rail project offer a once-in-a-lifetime opportunity to Loudoun County,” he said. “Please vote yes to support Loudoun’s continued involvement.”

Loudoun supervisors have a final scheduled work session Wednesday to discuss possibilities for financing the project. Options include the creation of a countywide commercial and industrial transportation tax, or the establishment of a special tax district for areas closest to the Silver Line.

Last week, Supervisor Matt Letourneau (R-Dulles) noted that the county could also choose to fund the county’s commitment to the project without raising taxes, by finding the money in the county budget. Loudoun would be responsible for more than $200 million in construction costs, plus an additional $11 million a year beginning in 2018 to support operating costs.

Much of the uncertainty surrounding Loudoun’s participation in the second phase of the project centers on a labor agreement proposed by the Metropolitan Washington Airports Authority. MWAA is expected to vote Wednesday on whether to move forward with the agreement, which has drawn the criticism of Loudoun leaders as well as Virginia Gov. Robert F. McDonnell (R).

If Loudoun supervisors approve the second phase, construction is slated to begin early next year.

Staff writers Anita Kumar and Dana Hedgpeth contributed to this report.