Northern Virginia legislators, seeking a key ally in their fight to preserve state funding for Metro, spent a day last month whisking one of the most powerful men in the Virginia General Assembly around the region.

In an unpublicized, red-carpet visit, state Sen. Edward E. Willey, the 75-year-old chairman of the Senate Finance Committee, was flown from Richmond to National Airport in a seven-seat private jet and toasted by a group of politicians and businessmen in the Crystal City corporate board room of the Charles E. Smith Cos. over coffee and danish. Driven in a limousine to one of the most handsome homes overlooking the Potomac in McLean, he was honored at a catered luncheon by 16 of the most powerful legislators and business leaders in the region, then spirited off to Dulles International Airport and home to Richmond by private jet.

"One treats Senator Willey with respect," laughed Earle C. Williams, president of the McLean-based BDM International Inc., who attended the luncheon.

"It was public relations on the part of the Northern Virginia delegation with a very senior and influential member of the General Assembly," said Sen. Joseph V. Gartlan Jr. (D-Fairfax), who attended both functions.

Willey, who has been in poor health the past year, was a key ally of Northern Virginia in the 1985 session of the General Assembly. Area legislators say he was instrumental in changing the allocation formula for state road-building funds to benefit Northern Virginia.

They also say his refusal to go along with a coalition of Southwest Virginia lawmakers killed their efforts to cut the state's $21 million annual subsidy for Metro transit. Willey, however, hinted during the session that he may not be so willing to support Metro during the 1986 session when Gov. Charles S. Robb, a Northern Virginian and a Metro supporter, is not in the governor's office.

The principal purpose of "Ed Willey Day" -- as some who attended the day's functions are calling it -- was to thank Willey for his support in the past battles.

Northern Virginia legislators say they hope Willey's whirlwind tour of the area July 2 will nudge him toward continued support for Metro.

"Anything you can do create a good feeling toward our area, which has not always had the best feelings from downstate in the past is worthwhile," said Del. Gladys B. Keating (D-Fairfax).

"I'm sure he was not influenced just because he was invited up for coffee and rolls and lunch," said Gartlan. "But it helped him meet some of the . . . very significant players in the Northern Virginia scene."

Reached at his Richmond office yesterday, Willey refused to discuss his stance on continued funding for Metrorail. He said he enjoyed his day in Northern Virginia last month, which he called "a social occasion."

"I just had a pleasant day with a lot of nice people," said Willey, who has served in the legislature since 1952. "That's it. No politics."

Among the nice people were Sen. Clive L. DuVal II (D-Fairfax), the leader of the region's delegation to the General Assembly; a half-dozen key state legislators from the region; John T. (Til) Hazel, a prominent lawyer and developer in Fairfax; and the presidents of a handful of large local businesses and the head of a bank.

The luncheon, furnished by a Georgetown caterer, was at the home of James T. Lewis, the head of a real estate concern and a law firm and president of Tytran, a group of business leaders seeking transportation improvements in the Tysons Corner area.

Some who attended the functions say they detected wholehearted support on Willey's part for Metro. But others were less sanguine.

"If there's one thing I've learned in the legislature," said Gartlan, "it's not to take Ed Willey for granted about anything."