A Metro finacing proposal that would place an additional 4 percent tax on gasoline purchases in Northern Virginia was approved today by a key committee of the Virginia legislature, in a move some area lawmakers say could damage the propects of Gov. John N. Dalton's statewide transporation tax proposal.

The measure, sponsored by Del. Warren Stambaugh (D-Arlington), would place squarley on Northern Virginians the state's responsibility for providing the stable funding base necessary before Metro can qualify the federal guarantees of $1.7 billion.

Stambaugh's bill was approved 17 to 2 by the House Finance Committee, with Del. John S. Buckley (R-Fairfax) the only Northern Virginia voting to oppose it.

"The timing [of the committee vote] was very bad," said Del. Gladys Keating (D-Arlington). "Probably the governor's proposal should have been considered first. It's a more proper vehicle, and would have been better for Northern Virginia."

Dalton's statewide four-cents-a-gallon plan would make Metro a beneficiary of funds raised for highway construction and maintenance, and spread funding responsibility throughout the state. That proposal is not scheduled to come before the committee until Friday. Some Northern Virginia delegates said privately today that the timing of the two proposals was engineered by Finance Committee chairman Archibald A. Campbell (D-Wythe) in an effort to kill Dalton's idea.

Campbell, who has opposed Dalton's plan since it was unveiled a month ago, said allegations that he was trying to torpedo the statewide tax were "not true."

"We are just trying to give the people of Northern Virginia an opportunity to solve their problem," he said. Campbell is known to favor a regional approach to curing Metro's money woes.

Under Stambaugh's proposal, a 4 percent tax on gasoline would be leived in the five Northern Virginia localities if the governing bodies of those localities agreed.

Money raised from such a tax, which Stambaugh estimated at $31 million in fiscal 1981, would then be distributed directly to the Northern Virginia's Transportation Commission to pay for Virginia's portion of the Metro system.

The $31 million still falls short of Metro's projected Virginia cost of $35 million; but Stambaugh emphasized that figures for his proposal were only a rough estimate.

"I'm basing this on the assumption that the average price of gasoline for the whole fiscal year will be $1.20 a gallon. If the price goes over $2, we'll do much better."

Under federal legislation approved last year, Metro stands to receive $1.7 billion in federal money if Virginia, Maryland and the District can demonstrate they have a stable and reliable source of revenue for the system's operating costs. Stambaugh said his proposal offered an alternative to jacking up area real estate taxes, which he said had been rising faster than those in any other state jurisdiction.

Stambaugh told the committee that the Northern Virginia delegation favors a statewide approach to Metro funding, but urged that they approve his plan as a backup. "We would prefer the governor's proposal," he said. "I would prefer to vote on that today and mine some other time, but we don't have that chance."

The Northern Virginia legislator said he was "considering" asking that his committee-approved plan be postponed on the House floor several days so that it could be considered side-by-side with Dalton's plan.

John F. Herrity, chairman of the Fairfax Board of Supervisors, said he would feel more comfortable if his jursidiction could have some "control" over its own Metro funds -- the country's second largest expense behind education.

But Stambaugh, miffed at Herrity's comments, said it would be hard to earmark -- jurisdiction by jurisdiction -- what is supposed to be a regional tax.

"I'm getting a little tired of coming here and trying to get my colleagues on this committee to give you $31 million in help, and you're telling us you don't want this help," said Stambaugh. "It doesn't make our job any easier."