Virginia Gov. John N. Dalton, in a surprising reversal of a long-held position, has decided he will approve an increase in the sales tax in Northern Virginia to help pay the region's operating costs for Metro.

The Republican governor has already proposed a statewide sales tax on wholesale gasoline prices to help raise money for highway and Metro subway construction. Now, he as assured several area legislators that he will no longer oppose the local tax measure to provide additional financial aid for the transit system.

"He wants to get everything done that needs to be done [for Metro]," Dalton spokesman Paul G. Edwards said today. "He did oppose the local sales tax last year. He's not opposed to it this year." But neither, Edwards stressed, has the governor embraced the local option tax as a good idea. Rather, he is deferring to the wishes of the region's political leaders.

Combined state and local sales taxes now total 4 percent. The new regional levy would boost the total to 5 percent.

Dalton's turnaround on the regional sales tax issue could represent an empty victory for the Washington suburbs. Many legislators say they doubt the General Assembly is ready to approve more than one major tax measure during the current session.

"We're going to have enough trouble passing the gas tax," warned Del. Warren G. Stambaugh (D-Arlington). "I think it's unrealistic to expect the legislature to help Northern Virginia pay for both its capital and operating costs for Metro."

Despite such concerns, most members of Northern Virginia's 27-member Assembly delegation are prepared to again introduce and press for some version of the one percentage point tax increase that has failed to pass here during the previous two sessions.

Supporters of the legislation -- who differ on whether the tax should first be approved by Northrn Virginians before becoming effective -- say the region needs the additional tax money to satisfy a federal requirement that Virginia provide a stable and reliable revenue source for Metro operations.

Dalton's gas tax proposal could raise about $24 million over the next two years for Metro construction costs but would not -- area lawmakers say -- address the system's mounting operating costs, expected to total $38 million in the Virginia suburbs by 1981.

Dalton is said to believe his gasoline tax measure will free local funds for use in defraying some of Metro's operating cost.

By linking the proposal to Metro and by withdrawing previous objections to the seperate sales tax bill, Dalton is expecting strong Northern Virginia support for his gasoline sales tax. The Dalton measure has not received strong support from legislators in any other region and could face an uphill fight in the Assembly.

The governor's major concern, according to an aide, is that the General Assembly not look upon the two pieces of legislation as equal. Only the gasoline tax has Dalton's wholehearted backing.

Even as they worried about the local sales tax's chances of passage, Northern Virginians are divided on what form the Metro aid legislation should take.

State Sen. Wiley F. Mitchell Jr. (R-Alexandria) said he would shortly introduce a sales tax bill that would require approval by voters in each of the region's political jurisdictions.

"I don't feel it will pass in the General Assembly without a referendum provision," Mitchell said. "And I think that on something this important, the people of Northern Virginia should be given a chance to vote.I'm sure they'd approve it overwhelmingly."

Mitchell said that Dalton had even "encouraged" him to sponsor the bill.

Other Northern Virginia legislators are expected to introduce the bill without any referendum provision -- as recently recommended by the Northern Virginia Transportation Commission.

"The delegation is not quite clear on where it wants to go on this, but I know I won't vote for a sales tax if it has a referendum in it," said Stambaugh, the senior Northern Virginian on the House tax-writing committee. "I don't think we should condition something so important to Northern Virginia on the whims of an election. That's why people elected us to represent them -- the monkey's on our backs."

Stambaugh said approval of the gasoline tax measure is far from a certainty. The "highway department has been riding high in the state for a long time and it's made a lot of enemies who don't want to see it rolling in money."

Senate majority leader Hunter B. Andrews (D-Hampton), echoing the views of numerous legislators from Northern Virginia and around the state, said chances for passage will depend on how the money it raises will be distributed.

"It should not be a partisan matter because I think we all understand the needs of Metro, the needs of the urban areas and the needs for secondary roads," he said.

Andrews said the regional sales tax bill might get through the Assembly more easily if it had "a sunset provision" -- a clause killing the tax after a certain number of years.

Northern Virginia legislators previously have managed to win support of a regional sales tax for Metro in the Senate, but the bill died in House committees in 1978 and 1979.