A powerful Virginia legislator threatened yesterday to eliminate the state's $21 million subsidy for Metrorail unless Northern Virginia legislators accept a major road-funding proposal that officials say would do little for Fairfax County.

"If Northern Virginia lawmakers expect to get any money for Metrorail, they better damn well support it," said state Sen. Edward E. Willey (D-Richmond), the chairman of the Senate Finance Committee.

Northern Virginia officials said the 75-year-old Willey's blunt remarks in an interview yesterday, were a sign that state funding for Metrorail will be held hostage to keep Northern Virginia from working against bills that would benefit other areas of the state.

Although Willey rejected the suggestion that he is offering a tradeoff, he assailed the Northern Virginia legislative delegation for expecting too much.

"That's always been the problem with Northern Virginia," Willey said. "They want to come down here Richmond and vote every damn which way and change the whole damn world. If these people would vote right, there's no reason we can't help them with their problem. They've got to reciprocate. That's the way things work down here."

In July, Northern Virginia legislators, seeking Willey's help in preserving state funding for Metrorail, gave the senator a tour of the region.

Willey was flown here in a private jet and driven in a limousine to a mansion overlooking the Potomac River for a luncheon in his honor with 16 of the most influential officials and corporate leaders in Northern Virginia.

"Probably didn't give him a strong enough" drink, said Fairfax County Board Chairman John F. Herrity. "He's probably sobered up by now and said, 'What did these people do to me?' "

Willey's remarks also may cast a shadow over the annual meeting of the Virginia Association of Counties, which begins tomorrow in Hot Springs, Va., not far from Lexington.

Northern Virginia officials and legislators called Willey's threat the first shot in what is likely to be a drawn-out battle over road funding, pitting the state's populous localities against its rural areas in the 1986 session of the state legislature.

"It's an opening salvo," said Fairfax County Executive J. Hamilton Lambert.

"It sounds to me like political posturing at this point," said state Sen. Edward M. Holland (D-Arlington). "I wonder who put a burr under his saddle."

Willey said he is drafting a bill that would raise $100 million for the state's primary roads. He would not say how the money would be raised, and denied rumors that it would come from an increase in the state's gasoline tax.

But Northern Virginia officials say they need more money for their region's clogged secondary roads, and would probably gain little from new primary road funds.

Early this year, Northern Virginia won a major victory in the legislature by redrawing the road-funding formula to channel more money to secondary roads. If Willey's bill were approved, they said, it would undo that victory.

"It would certainly alter" the new formula, said state Sen. Clive L. DuVal (D-Arlington), head of the 29-member Northern Virginia legislative delegation. "It is not the kind of thing we would be willing to go along with on a long-range basis."

He also said he thinks Northern Virginia, with the help of urban allies from the Hampton Roads area, has the clout to block -- or at least significantly modify -- an unfavorable bill. "No tax revenue bill will go through without our approval . . . . We hold a very strong hand."

State Del. Vivian E. Watts (D-Fairfax), while saying repeatedly that she was not eager to tangle with Willey in public, said, "What Sen. Willey is talking about is horse trading."

One of the key items on the agenda for the Virginia Association of Counties meeting is a proposed statewide bond for road improvements, probably for the 1987 ballot. The organization's 94 county members are said to be badly split on how to structure such a proposal, with Northern Virginia and other populous areas favoring a bond that would fund secondary-road improvements, and rural counties insisting on a bond to pay for primary-road improvements or for a specific list of road projects.

The bond, if it is approved by the state legislature, would probably be for at least $500 million, officials said. The state's total road needs are said to be in the range of $16 billion.