Gubernatorial candidate Andrew P. Miller yesterday proposed using Virginia highway trust funds and, if necessary , general tax reveunes, to finance completion of the Metrorail subway system into Northern Virginia, but he attached several key conditions to his offer.

"The state has an obligation to Northern Virginia to recognize that there is a unique (transportation) situation here and that the state should act in a responsible manner," Miller said during a luncheon with Washington Post reporters and editors.

Although his promise to help secure state funding was qualified, it appears to be the most precise offer of state aid that any of the three major gubernatorial candidates in Virginia has made this year. It is in sharp contrast to the attitude of Republican Gov. Mills E. Godwin who once vetoed a $10 million Metro appropriation and later approved a conditional grant with serious misgivings.

Henry E. Howell, Miller's opponent in the June 14 Democratic Primary, has called for "more adequate capital aid" for Metro. John N. Dalton, the expected Republican nominee, has said he would have apaproved the $10 million grant Godwin vetoed and believes the system should be studied further.

Under the Miller proposal, the state would be obligated to pick up capital for completing the portions of the system that are curently unfunded and under review by a regional task force. If Northern Virginia leaders can agree on what portions of two disputed routes should be completed and can develop a plan acceptable to the legislature, Miller said he would support such a proposal.

But he made clear that he would expect to have a hand as governor in drafting any such proposal before it went to the General Assembly and involved in the planning the routes.

Currently under review by Metro are proposals to extend a line from the Ballston Station in Arlington west to the Springfield-Franconia area of Fairfax County. If these lines are completed as initially proposed in the 100 mile Metro system, Virginia localities would be obligated to pay $87.2 million more than their share of the costs, according to William Boleyn, assistant general manager for Metro finance.

However, it is generally believe d to be unlikely that the current study will recommend that both lines be completed as the original 100 mile plan called for.

"I do not think that the state should be in a position of making the ultimate decision" as to the system's final shape in Virginia, Miller said. "But I do think that the state should be a participant in the decision making that leads to a final decision."

"It's a question of trying to work out a reasonable package in which the state would commit itself to underwrite the remaining capital costs," he said. "But the state is not going to go into this blindly and say, 'whatever you guys want, the state is going to pay the capital costs,'" he continued.

Funds for the construction could come from a portion of the state's highway construction trust fund currently earmarked for mass transit purposes and if necessary, from the state's general tax sources, Miller said.The state funds would be used to match federal dollars at a ratio of one to four.

Miller, a former state attorney general, disclosed his views as he discussed his campaign proposal to remove mass transit programs from the State Department of Highways and Transportation and place them in a new, separate agency.

Earlier at a Richmond news conference, Miller unveiled endorsements from 62 black ministers from around the state - ministers his campaign said could deliver him 25 to 35 per cent of the black vote in the primary.

Such a black vote for Miller would cut significantly into a block of voters generally conceded to be vital to the campaign of Miller's primary opponent, former Lt. Gov. Henry E. Howell Jr. of Norfolk.

The Rev. Curtis Harris of Hopewell, one of eight ministers speaking for the group, said it was the first time in Virginia political history that as many ministers have banded together to help elect any statewide candidate. Together, he said, they represent churches with a total membership of about 50,000.

Howell earlier received the support of Virginia's largest and most influential black political groups, including the Richmond-based Crusade for Voters, the Black Caucus (also based in Richmond), and the Concerned Citizens of Norfolk.