Virginia Democratic Gov. Charles S. Robb attacked Republican Sen. John Warner today for sending out a political fund-raising letter that Robb described as "an affront to the intelligence of the average citizen."

The Warner letter suggested that Alexandria lobbyist William G. Thomas, a close friend of Robb's, was one of several potential Democratic candidates for Warner's Senate seat next year who would be supported by "liberal special interests" and were "far more liberal on national issues than the great majority of Virginians." The letter was recently mailed out to about 40,000 Republicans around the state.

"I really do think it's not befitting the credibility of a campaign or the candidate to make a statement that is not true," responded Robb, when asked about the letter at a news conference today.

Warner's office refused to make any comment on Robb's remarks. But the governor's unusually partisan comments were quickly interpreted by some Democrats here as a means of boosting Thomas, a former Robb fund-raiser, at a time when the party is once again having trouble finding a U.S. Senate candidate. Many of the state's leading Democratic officeholders--such as Senate Majority Leader Hunter B. Andrews and Robb himself, have ruled themselves out.

The only Democrat to express outright interest to date is former delegate Edythe Harrison of Norfolk, who met with Robb on Monday and said afterward she planned to announce this fall. "I'm the only candidate," Harrison said.

On Monday, Thomas initiated the flap over the Warner letter after he inadvertently received a copy in the mail. In a stinging response released to the press here, the Alexandria lawyer called on Warner to identify which " 'liberal special interests' are involved with my potential candidacy.

"I don't think anyone has ever categorized me that way liberal in my life," said Thomas in a telephone interview today. "It's a typical Republican scare tactic letter."

Thomas, who has represented the Mobil Oil Corp., the Condominium Developers Association and other business interests before the General Assembly, said that he has discussed his potental candidacy with Robb "a number of times" in the past few months. While he insisted that he has not yet made up his mind about running, he acknowledged that his exchange of letters with Warner "shows that I've got some interest."

The chief problem that many Democrats have cited in backing off a race has been an inability to raise a large enough campaign war chest to do battle against Warner. A Warner assistant said today that the senator, who was first elected in 1978, has already held a number of campaign fund-raisers around the state and raised about $400,000 for next year's race.