Virginia Gov. John N. Dalton last night reaffirmed his opposition to increasing Northern Virginia's sales tax to fund the Metro subway system, but refused to say whether he would veto an increase if one is approved by next year's legislature.

Dalton said the Northern Virginia jurisdictions served by Metro will receive about $10 million annually in state transportation funds, plus another $20 million in indirect funding under a new law increasing state appropriations to localities. He insisted the total would be enough to meet Metro's operating deficit over the next 10 years.

The Republican governor's contention was immediately challenged by State Sen. Adelard L. Brault of Fairfax, Democratic majority leader, and a proponent of a 1 percent sales tax increase at last year's General Assembly session.

The problem of paying for Metro long has been a source of contention between Northern Virginia and the rest of the state. Attempts to increase the tax to 5 percent from its present 4 percent level were narrowly defeated in the last two legislative sessions.

Dalton, who made his comments during an hour-long question and answer program over WJLA-TV (Channel 7) said last night he was committed to helping localities fund Metro's operating deficit over the next 10 years, but not with the sales tax.

"If we raise the sales tax to 5 percent in one area of Virginia, we will inevitable have to allow other areas to do it too," he said.

Brauly noted in a telephone interview after the broadcast that the U.S. Senate that would earmark $1.7 billion to finish building the system. He called on the Carter administration to support the bill, which has passed the House of Representatives.

Dalton's appearance last night, his first trip to the area since when the gasoline crisis here was at its peak. in June, is a prelude to two months of campaigning in Northern Virginia for GOP legislative candidates. Press Secretary Paul Edwards said the governor expects to make at least five political trips to the area next month and nearly as many in October.

Dalton answered citizens' questions on a variety of subjects, both by telephone and from an audience that was televised at an Alexandria fire house. Among his comments:

Dalton said he would not lift the odd-even gas rationing program for Northern Virginia until local officials request that it be removed. "So far no one has," he said.

He repeated his support for capital punishment and said he had no plans to commute the sentences of seven persons presently on death row awaiting the outcome of their appeals.