Gov. Robert F. McDonnell has amended the state budget, possibly withholding money for the authority that operates Reagan National and Dulles International airports until Virginia’s two new members are seated on its board.

State lawmakers said Thursday that the change could hold up the state’s $150 million contribution to Metro’s multibillion-dollar planned extension to Dulles, but officials with the Metropolitan Washington Airports Authority say McDonnell’s administration has assured them it does not.

Finance Secretary Richard D. Brown said Thursday that the governor’s amendment was designed to force the board to seat his two nominees. The board has resisted the appointments for months.

“Our intent is not to hold up the $150 million,” Brown said.

McDonnell (R) added the amendment to the two-year budget on Saturday. The budget runs through June 30 and includes language that directs bond proceeds to the state’s $150 million contribution for the rail project.

Democratic legislators plan to fight the change Monday when they return to the state Capitol to take up McDonnell’s nearly 100 budget amendments.

“I think it’s inappropriate for the governor to do that,’’ Sen. A. Donald McEachin (D-Richmond) said. “That money is needed.”

Virginia’s contribution to the second phase of the Silver Line has consumed Richmond for weeks, with Senate Democrats holding up the new $85 billion budget as they tried to secure an additional $300 million for the Silver Line.

The state Senate killed the proposal after McDonnell said he would not spend more than $150 million.

Transportation Secretary Sean Connaughton had also threatened to withhold the money without some other management changes at the authority. Virginia has been reluctant to put in money on the second phase of the Silver Line until the authority drops an incentive in a pro-labor work agreement for contractors bidding on the project.

Connaughton said that the budget amendment has no bearing on the $150 million and that he has repeatedly told MWAA officials that.

He called assertions that there is a connection “complete fabrications.”

Connaughton said legislators and legislative staff are wrong. “They don’t know what’s going on,” he said. “They’re idiots.”

The first phase of the $6 billion Silver Line is under construction from Falls Church to Reston and is expected to be completed in 2013. Construction on the second part of the project, which will run to Dulles and farther into Loudoun, is scheduled to start in the spring.

MWAA board members say that they believe the budget language does not affect the $150 million they expect to get from Virginia and that Connaughton told them there is “no linkage” between seating the new members and getting the money.

Jack Potter, MWAA’s chief executive, said he spoke to Connaughton on Wednesday, and he “assured me that this has nothing to do with the $150 million.”

Staff for both of Virginia’s legislative money committees said they believe this change could hold up the $150 million.

“That’s my read of it,’’ said Robert Vaughn, staff director of the Appropriations Committee.

Potter said “this provision seeks to accelerate the resolution of the differing opinions” about the timing of additional board members and that in “all likelihood” the case will end up going to the courts. McDonnell hinted last month that the state may sue to get Virginia’s members seated on the board.

The General Assembly passed a bill this year that would allow McDonnell to seat his appointees, but it delayed the seating until July 1. The appointments come out of congressional legislation introduced by Rep. Frank R. Wolf (R-Va.) to expand the board from 13 to 17 members.

“It sounds like it’s overriding an interstate compact,’’ Sen. Janet D. Howell (D-Fairfax) said. “If it is, he is truly putting the money in jeopardy.’’