About 200 of Northern Virginia's most powerful developers and business leaders, worried that they are being blamed for the region's transportation problems, gathered Tuesday night at a sumptuous Potomac River estate to plan a public relations counteroffensive.

The $2,500-minimum per person, invitation-only affair at Merrywood, the 26-room McLean mansion where Jacqueline Kennedy Onassis grew up, was one of the most elegant and high-priced fund-raisers ever held in Northern Virginia, according to participants. Reporters were barred from the estate, which is now owned by developer Alan I. Kay.

According to most estimates, the affair raised at least $300,000, and possibly a good deal more.

The business leaders, calling themselves the Northern Virginia Transportation Alliance, feasted on Norwegian smoked salmon, roast tenderloin of beef, crab mousse and marinated scallops. They also took in several speeches stressing, in the words of developer Thomas M. Hirst, one of those in attendance, "that we want to be regarded as good citizens who are making this a better place to live and who are putting a good deal of money into setting the record straight."

Guests also heard two radio advertisements and watched a television commercial that will be broadcast this month, the most critical time in the political campaigns for the Fairfax County Board of Supervisors. The ads, paid for by the alliance, stress the regional nature of transportation problems and solutions, and urge voters in the Nov. 3 elections to assess candidates according to their ability to unsnarl the region's roads, several guests said.

Despite the alliance's vigorous assertions that it is not trying to influence the voting, some Democrats are skeptical.

Many of the leading members of the alliance are major contributors and fund-raisers for board Chairman John F. Herrity, a Republican, who is seeking his fourth term. One of the few Democratic members of the alliance, state Del. Dorothy McDiarmid of Fairfax, who is chairman of the powerful House Appropriations Committee, said yesterday that she did not receive an invitation to Tuesday night's gala.

Democrats also point out that the alliance, which for a time tried to enlist former Virginia governor Charles S. Robb as its honorary chairman, did not accept Robb's recommendation that advertising be postponed until after the election in order to avoid the appearance of meddling in politics. Robb did not join the alliance.

Janice Spector, spokeswoman for Supervisor Audrey Moore of Annandale, the Democrat challenging Herrity, said: "If indeed they are nonpartisan, I don't know why they're doing a media blitz a month before the election to talk about an issue that is already being discussed in a very heated political race."

"What we're doing is responsible and is the appropriate way to . . . move forward on this problem," alliance President Linda Wright said. "We want the transportation problem solved."

Although the group has said its primary purpose is to educate the public about transportation issues, two reporters from The Washington Post were denied entrance to the fund-raiser, which had been widely publicized.

"Please leave," said Edward S. DeBolt, a Republican pollster and the alliance's media consultant. "You know what the price tag is here."

Several alliance members and guests said yesterday that they were angry and bewildered when DeBolt announced at the fund-raiser that he had escorted the two reporters from the premises. "Why not just open it up . . . and say, 'Here it is'?" said James M. Irving, a developer and Herrity fund-raiser who attended. "I don't know why the press should have been excluded."

Said Hirst, a Democrat: "I was really outraged. Why in the world wouldn't they welcome the press? Everything was on the up-and-up. It's so peculiar that a group that is in the business of improving its public image would not let the press in."