The Metropolitan Washington Airports Authority is expected to cast a pivotal vote Wednesday on the future of Metro’s Silver Line, barely a day after the project’s supporters and opponents faced off in Leesburg over bringing the rail line to Loudoun County.

A union-friendly incentive included in plans to build the second phase of the Silver Line has emerged as a potential deal-breaker after drawing sharp criticism from Loudoun County leaders and Virginia Gov. Robert F. McDonnell (R).

McDonnell and the Loudoun County Board of the Supervisors have said that if the project labor agreement, or PLA, is not taken off the table, they could withhold about $350 million pledged for the second phase of Silver Line construction.

At a public meeting Monday, Board of Supervisors Chairman Scott K. York (R-At Large) urged the MWAA to abandon the proposed agreement, calling it the “right thing” to do.

Most of the more than 100 people who spoke expressed support for the Silver Line extension.

The board, which on Wednesday night has a work session on the Silver Line, has until July 4 to decide whether to uphold its commitment to pay about $200 million toward the construction of the Silver Line.

As the deadline nears, proponents and opponents have stepped up their rhetoric.

Last week, the conservative group Americans for Prosperity made thousands of robo-calls, urging Loudoun residents to oppose the extension and saying that county taxpayers should not have to foot the bill for the project.

And area business leaders and labor organizations have been running ads in local papers and appealing to Loudoun supervisors to consider the potential economic benefits of bringing Metro into the county.

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 Republicans’ objections to the MWAA’s planned labor agreement.

In a letter this week to U.S. Transportation Secretary Ray LaHood, McDonnell reiterated his position, saying that the commonwealth would provide $150 million in additional funding if the MWAA agreed to pull the union-friendly incentive from the labor agreement.

At Monday’s meeting, an opening lineup of about a dozen Silver Line opponents reminded officials of campaign promises of fiscal responsibility and lower taxes.

“It’s not the project I object to, Mr. Chairman,” Del. Robert G. Marshall (R-Prince William) said to York. “It’s the cost of the project.”

Marshall, who is running for the U.S. Senate, questioned whether the county should trust the MWAA’s management of the project.

“You are going to have no control over these costs,” he said. “I urge a ‘No’ vote.”

Todd Morrison, a Purcellville resident, said he hoped that the supervisors would have the “courage” to turn away from the project.

“On July 4, may all Loudoun County residents be able to celebrate their declaration of independence from Metro,” Morrison said.

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

Opponents of the project were outnumbered by supporters, many of whom wore green t-shirts declaring “Loudoun Rail Now.” A steady stream of residents and business leaders voiced fervent support for the rail extension, reminding Loudoun’s all-Republican board of its pledge to help attract new businesses and expand the county’s commercial tax base.

Many speakers described the Silver Line as a forward-thinking investment in Loudoun’s next generation of workers and residents.

Grafton DeButts, a founding member of the Loudoun Young Professionals initiative, said that Metro is 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 sentiment. She said that she moved to Sterling rather than the District but that it was a difficult choice.

Tony Howard, president of the Loudoun County Chamber of Commerce, was one of many speakers who referred to the Dulles rail project as a “once-in-a-lifetime opportunity” for the county.

Speaking on behalf of the chamber’s 1,200 member businesses, Howard urged the board to support Loudoun’s continued involvement in the project.

The board’s work session Wednesday night will weigh options for financing the county’s promised contribution to the construction of the Silver Line. Among the possibilities are creating a countywide commercial and industrial transportation tax or establishing a special tax district for areas closest to the line.

Last week, Supervisor Matt Letourneau (R-Dulles) noted that the county could also choose to fund the county’s commitment to the project by finding the money in the county budget instead of raising taxes.

If supervisors approve the second phase of the Silver Line, construction is set to begin early next year.

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