Transportation Secretary Brock Adams committed the Carter administration for the first time yesterday to "the goal of completing the 100-mile" Metro subway system, but stressed that the commitment depends on an up-front guarantee of millions of dollars from Washington area governments.

If Metro's local partners can raise 20 percent of the remaining $3 billion needed to complete Metro and can guarantee that there will be money to operate the system, Adams said in effect, the federal government will be willing to provide the remaining 80 percent of the construction costs.

Adams' declaration came less than one hour after the Metro board had been formally briefed on a complex proposal to complete the system and solve Metro's long-term financial difficulties.

Among other things, the proposal estimates that a full Metro subway will cost either $6.7 billion or $6.9 billion - the highest numbers ever officially released. It also suggests, however, that there are no cheap alternatives to solving the area's traffic and transportation problems.

Adams praised the Metro board proposal - which will not even be officially forwarded to federal, state and local governments until next week - as fulfilling the requirement he laid down in a series of letters to Metro last year.

"The federal government agrees with the goal of completing the 100-mile system over the next several years," Adams said. "Now we have to move together through a period of interim negotiation."

There are several points of difference between the Metro board's proposal and Adams' initial response. While those points are substantive, they do not rank in importance with the overall question of whether the federal government supports the entire system.

Area officials seemed generally pleased with Adams' response. "We are now arguing about the best method instead of the goal," Metro General Manager Theodore C. Lutz said. "For the last two years, we have been arguing about the goal."

Adams made clear both during the press conference and in a brief interview afterward that Metro must find a way to guarantee that local money for constructing and operating Metro will be available before the federal government commits itself to more huge Metro expenditures.

"The Metro board must put on line their financing system," Adams said. "They need to face up to payroll taxes, sales taxes - some kind of mechanism to fund this system and provide the operating subsidies that will be needed."

The Metro board draft proposal contains a number of options that raise varying amounts of money without recommending a specific form of tax for the entire region. It is generally agreed that each of the three major Metro partners - Maryland, Virginia and D.C. - must decide on its own dedicated tax source for Metro, just as other localities have been required to get federal funding for mass transit projects.

Metro is financed largely by property taxes today and "we think it is highly desirable" to remove Metro from the property tax burden, Metro board chairman Joseph S. Wholey told the board yesterday. "If any major jurisdiction were to step out of Metro (because of high tax rates), they system would grind to a halt," Wholey said.

That very possibility is known to be a major concern to federal officials studying Metro and is thus the underpinning for the federal insistence that Metro guarantee its operating budget in advance.

Under the latest Metro board projections, the annual Metro operating deficit will be $328 million in 1990 with the full 100-mile system completed. In addition to paying that bill, area governments will need to come up with at least another $800 million in construction money to finish the system. That will probably come from general obligation bonds, which must be approved in elections in some jurisdictions.

Ideally, in the view of most Metro board members, a dedicated tax should cover both the operating deficit of the subway and bus systems, plus the debt service (interest and principal) on construction bonds.

Metro has presented proposals showing the amounts of revenue that several kinds of taxes would raise along with tables estimating the total Metro bite on local budgets in the future.

Wholey and other board members are scheduling a series of briefings to explain their proposals and the need for earmarked tax money to Maryland Acting Gov. Blair Lee III, Virginia Gov. John N. Dalton and Mayor Walter E. Washington. Meetings with local city councils and county boards as well as with influential congressional committees, also are scheduled.

Adams indicated his pleasure with the work Metro has done to date by promising to support $275 million in federal money for Metro construction in a supplemental appropriation for fiscal year 1979. Metro had previously been shut out of the President's FY '79 budget.

Adams met with Carter on the Metro question Wednesday and stressed that the "goal of completing 100 miles" had administration-wide support. "We are not going to request any more studies," Adams said. "The system has been studied and reaffirmed."