RICHMOND, MARCH 4 -- Andrew F. Wahlquist, a longtime top aide to U.S. Sen. John W. Warner (D-Va.), today became the second Republican to announce that he wants to take on Democrat Charles S. Robb in November for the seat being vacated by the state's junior senator, Paul S. Trible Jr. (R-Va.).

Until last Friday, when Gil Faulk, a business executive and attorney who moved to McLean last year from New York, announced his candidacy, the GOP had been unable to find anyone to challenge Robb, who is expected to be unopposed for the Democratic nomination.

Many Republicans have complained that Trible, after raising about $2 million for a second term, backed out of the race rather than risk being defeated by the popular former governor. Trible has denied the assertions, saying the Senate was not effective and that he wanted to spend more time with his family.

Wahlquist, 48, said the issue at stake in his candidacy is not Robb, but Republican control of the Senate, which the party relinquished two years ago.

"I am not running against Chuck Robb," Wahlquist said in a speech he delivered today at four locations around the state, including Alexandria. " . . . We need to stop the liberal attempt to roll back the accomplishments of the Reagan administration."

In a jab at Robb, Wahlquist made "three promises": He will not run for vice president this year; he will not run for president in 1992, and "I will never cast a vote that would help elect Bobby Byrd, Dan Inouye or Ted Kennedy majority leader of the Senate."

Wahlquist, the Virginia coordinator for the Bush for President campaign, has been a partner in a McLean public affairs consulting firm and affiliated with a Fairfax City developer since resigning as Warner's administrative assistant in June 1986. He served 7 1/2 years as Warner's administrative assistant.

Warner declined to endorse Wahlquist, but had nice things to say about his former assistant, describing him as "an outstanding Virginian who has a special knowledge of the inner workings" of the Senate.

The state's senior senator said that to "foster needed competition," he is "committed not to give my personal endorsement to any candidate" before the state GOP convention June 10 and 11 in Roanoke.

Other Republicans considering the race are former 4th District representative Bob Daniel, U.S. Customs Commissioner William von Raab and Maurice Dawkins, a Washington lobbyist who has been active in trying to enlist minorities to the GOP and unsuccessfully sought his party's nomination for lieutenant governor three years ago.

A spokesman for von Raab, 46, who has a farm in Orange County, Va., and a town house in Alexandria, said today his boss is "giving serious consideration" to jumping in. He first would have to give up the Reagan administration post he has held since 1981.