Transportation Secretary Drew Lewis declined an invitation from top area political leaders yesterday to commit the Reagan administration to supporting completion of the full 101-mile Metro subway, but said he is "supportive of the system."

Lewis told one governor, three senators, four congressmen, the D.C. delegate and two Metro board members "it is clear this is a worthwhile program." However, he said, "it is a cold fact that if we cut the budget there will be delays in completing this system as well as others."

Sen. John W. Warner (R-Va.), who had collected the throng of political talent in the Senate's Russel Office Building, invited Lewis to go further. "I hope you can see your way to support the 101 miles," Warner said.

Lewis ducked. "I'm at a little bit of a disadvantage," he said. "I've only been on the job 30 days."

Nonetheless, Warner and other Metro backers took comfort from words such as "supportive" and "worthwhile," because they are the most positive things they have heart from the Reagan administration to date. Budget director David Stockman's famous "black book" was unkind to even the concept of rail transit and carried this sentence:

"Considerable local logrolling is generally necessary to generate sufficient local support for a new system. This leads to excessive mileage (e.g. Washington Metro)."

Paranoia levels among Metro's backers shot skyward. Yesterday they organized one of those impressive displays of regional unity to engage in a little more logrolling. Even Sen. Harry F. Byrd Jr. (Ind-Va.), who has not attended a Metro pep rally in years, showed up.

"I'm not here to suggest that any project be exempt from sacrifice," Byrd said, but he called Metro important to the future of the metropolitan area and said "we hope [construction] could be pushed along at the best rate possible."

Virginia Gov. John N. Dalton, a Republican, was there. "We're willing to take our share of the cuts," Dalton said, "but don't abandon the program." Dalton was lukewarm for Metro when he ran for the statehouse three years ago, but he is thought to be considering a race for Byrd's Senate seat of Byrd retirees next year, and a strong stand for Metro is regarded as vital by politicians of both parties if they are to carry Northern Virginia.

Maryland Gov. Harry Hughes sent his regrets (he planned to attend as recently as Tuesday). D.C. Mayor Marion Barry, who potentially has the most subway to lose if the Metro system is cut back, was absent. He sent his transportation director, Thomas Downs.

Barry said last night that he had been scheduled to be out of town, although he changed his mind at the last minute. "I don't think I have to be at all these meetings to show the city's support for Metro," he said. "We've put our money there. Our support for Metro is unwavering."

D.C. Del. Walter Fauntroy and City Councilman Jerry A. Moore Jr. carried the city's flag. "I'm particularly concerned about some of the [uncompleted] inner city routes," Fauntroy said. He called Metro important to inner city residents because it would provide "access to jobs and economic development . . . ."

Lewis also heard from Sen. Paul Sarbanes (D-Md.) and Reps. Michael Barnes (D-Md.), Marjorie Holt (R-Md.), Frank Wolf (R-Va.) and Stanford Parris (R-Va.), plus Metro board chairman Joseph Alexander.

In a prepared text, Warner asked Lewis for a federal appropriation of $350 million in fiscal 1982. That is the amount Metro is using for planning purposes on a schedule that would bring completion of the system by 1991. However, Warner did not mention the figure when he talked to Lewis publicly. He said in an interview later tha $350 million "is a good target" and that he had eliminated it from his remarks in the interest of saving time.

Thirty-seven miles of Metro are in operation and another 23 are funded and all but assured of completion. The remaining 41 miles are in various stages of construction, design or planning.

Lewis and other Department of Transportation officials have consistently declined to explain what rail transit programs they will support and at what levels, although early budget figures indicate a total program that is substantially lower that this year's. Baltimore, Miami, Atlanta and Buffalo also have systems under construction. Several other cities, notably Detroit, Los Angeles and Houston, have ambitious plans well under way.

"We're trying to spread the funds," Lewis said, "and not overspend in one area to the detriment of another." At another point he said, "I'm a great believer in mass transit, particularly in the Northwest."