Virginia Gov. Robert F. McDonnell’s current position is clear: The state will not contribute to Metro’s planned extension to Dulles International Airport unless a union-friendly deal for contractors is dropped.

But McDonnell (R) hasn’t always taken such a hard line with the Metropolitan Washington Airports Authority, the quasi-public agency overseeing the $6 billion construction of Metro’s new Silver Line to Dulles Airport and further into Loudoun County.

Just months ago, the McDonnell administration was negotiating with the MWAA over the terms of a mandatory project labor agreement for the second phase of construction, according to interviews with legislators and a review of correspondence to and from the governor’s transportation chief.

But facing pressure from conservative Republicans, McDonnell and his staff pulled back the administration’s support for a project labor agreement, or PLA, legislators from both parties say.

Now, the governor says that unless the MWAA abandons a PLA for the second phase of the rail line, the state will withhold a promised $150 million contribution.

Del. Joe T. May (R-Loudoun), chairman of the House Transportation Committee who shepherded McDonnell’s MWAA-related legislation this session, called the initial language the McDonnell administration negotiated a “good compromise.”

May said he has been baffled by the administration’s shift on project labor agreements, likening the situation to the famous comedy routine by Abbott and Costello “Who’s on First?”

“There has been enormously changing ground rules,’’ he said. “This has been one of the more confusing issues.’’

Project labor agreements typically ensure certain wages and working conditions while barring strikes and providing managers with flexibility in making work assignments.

Such pacts are common in state and federal projects, and a voluntary PLA has been in place for the first phase of the Silver Line construction.

The fight over a mandatory PLA for Phase 2 is the latest crisis to confront Metro’s effort to extend the rail system.

This week, the MWAA board of directors is expected to vote on whether to press ahead with the labor agreement. So far, it has shown no inclination to drop it. The board’s decision could determine whether the Silver Line ever reaches Loudoun, where supervisors have said they won’t make an estimated $200 million contribution if the labor agreement goes forward.

The uncertainty surrounding what is one of the country’s largest infrastructure projects has all but ensured that it will be delayed.

Already, the latest debate has prompted planners to put off seeking bids from contractors to build the second phase of the Silver Line from Reston to Loudoun.

The first phase of the project, which runs through Tysons Corner to Wiehle Avenue, is expected to be finished in August 2013, about $150 million over budget. Planners had wanted to have bids out in early March to pick a contractor for Phase 2 but now say they are at a standstill until Virginia and the MWAA board work out a deal.

Careful wording

Not long ago, the path to completion seemed smoother.

A series of e-mails sent last year show that McDonnell’s point person on the Silver Line, Transportation Secretary Sean T. Connaughton, tentatively agreed to the project labor agreement after spending months negotiating language with MWAA officials.

The e-mails were provided to The Washington Post by people who are sympathetic to the MWAA and who have been frustrated that McDonnell’s shift has drawn little attention.

The dozens of brief notes exchanged over a period of four months and a subsequent memorandum of understanding among Silver Line contributors show that Connaughton and Jack Potter, the MWAA chief executive, worked carefully on the wording before agreeing to a set of principles that enabled a mandatory project labor agreement.

“Sean, are you okay with the principle below?’’ Potter asked July 27.

“Yes,’’ Connaughton responded.

In November, in an apparent sign of continuing progress, the MWAA asked Connaughton where to send a copy of the agreement for his signature.

But the sentiment soon changed. Connaughton never signed an agreement for a PLA for Phase 2. In February, amid mounting Republican criticism of the PLA for Phase 1, the General Assembly enacted a law intended to prohibit PLAs on the Silver Line project.

Connaughton did not respond to requests for comment for this article. McDonnell spokeswoman Taylor Thornley declined to answer questions and instead released a brief statement: “We have never supported a mandatory PLA. Virginia law now reiterates that position.”

Administration officials acknowledge that Connaughton had been negotiating. But they say it was for a voluntary PLA, not a mandatory one. Later, McDonnell sent a letter saying Virginia would not accept an agreement that violated the state’s right-to-work law. Such statutes prohibit a company and a union from signing a contract that requires workers to be union members.

But Potter said Virginia’s right-to-work law allows a mandatory project labor agreement. “I don’t think there was a comprehensive understanding of the right-to-work law.”

Settled, then not

For months, state and MWAA officials worked on a project labor agreement both sides could accept.

Connaughton’s attention to detail exasperated some MWAA officials. “This is getting ridiculous; he just doesn’t stop,” MWAA lawyer Phil Sunderland wrote to Potter on Nov. 16. But Connaughton’s persistence appeared to confirm that the McDonnell administration was on board.

It was on Nov. 16 that the MWAA board unanimously voted in favor of requiring a contractor to have a project labor agreement.

When MWAA officials sent an e-mail after the board vote to confirm Connaughton’s acceptance of the language of the deal, he responded, “just a couple nits,” and pointed out grammar and punctuation mistakes.

But on Dec. 30, the MWAA chairman, Michael Curto, received a letter from McDonnell. He wanted his MWAA board nominees to be seated and said he had concerns that the proposed contract would violate Virginia’s right-to-work law. That was a question that the MWAA thought it had settled after the office of state Attorney General Ken Cuccinelli II (R) advised Connaughton that project labor agreements did not violate Virginia law. The MWAA had adopted language suggested by the attorney general’s office.

McDonnell was facing heat from the conservative wing of his party. “They’re responding to the concerns of the legislature,’’ said Del. Robert G. Marshall, a conservative Republican from Prince William who is running for U.S. Senate and has been vocal on the issue. “He’s pulling back because Republicans are putting their foot down.”

Sen. Mark R. Herring (D-Loudoun) said McDonnell changed his mind on the PLA the same way he did when he initially agreed to give up to$300 million more   for the project.

“That’s not what they had agreed to,’’ Herring said. “It was an about-face. There are certain voices in the governor’s party who didn’t agree and caused him to disagree.’’

MWAA board member Bob Brown said Connaughton had negotiated a mandatory PLA for Phase 2.

“It’s very clear and the records showed that. We had a handshake deal,’’ he said. “We have had to constantly change things to accommodate this guy. He’s made more noise than anyone at the table.’’

In February, the MWAA board dropped its mandatory project labor deal and voted unanimously to provide incentives to prospective contractors who agree to follow a voluntary project labor agreement. But McDonnell says he does not want any PLA.