Freshman Fairfax County Supervisor Sandra L. Duckworth likes to portray herself as a political novice, but her 10th day in office yesterday found her chatting with President Carter in the Oval office about key county issues.

In Virginia, a state that prides itself on tradition and formality, Duckworth's sudden success in carrying the county's message brought her in for immediate criticism from the Board of Supervisors' most influential Democrat.

"It gives the appearance that she grabbed all the aces and left the room, leaving everyone else to play cards with no aces," claimed Martha Pennino (D-Centreville), who is beginning her third term as the board's vice chairman.

Duckworth, who is Virginia's National Democratic Committeewoman and often refers to herself as the "baby" of the Fairfax board, lobbied Carter to support federal funds for a regional project to generate energy from garbage, and thanked him for his backing on Metro funding for Virginia.

Pennino, who seemed to be smarting at an apparent slap to her authority, said she was "miffed" at Duckworth's "breach of confidence" in talking to President Carter about the energy plan.

"I certainly wouldn't want Mayor Barry to think we had violated his confidence," Pennino said, adding that Duckworth did not not go to the White House as a spokesman for the Fairfax County board. "He [Carter] might wonder: how far can I trust these people?" Pennino said.

Officials from Fairfax County and the District recently began efforts to fund a project that would produce electrical energy or steam from the incineration of solid waste and sewage sludge. The operation, which could cost as much as $300 million, would be eligible for up to 85 percent federal funding. It was this federal assistance that Duckworth was seeking from the president.

Duckworth, who has known Carter since 1974 and helped coordinate his presidential campaign in 1976, said her trip to the White House yesterday offered her a chance to represent county interests at the federal level.

"I have no idea whether I can have an impact at all," the Mount Vernon supervisor said before her three-minute meeting with the chief executive. "But nothing ventured, nothing gained, I feel."

Elected last November to her first term in public office, Duckworth is seen by local Democrats as a formidable force in partisan politics. She is often mentioned as a possible candidate for Congress if, as many local Democrats forecast, Sen. Harry F. Byrd (I-Va.) retires in 1982 and Rep. Herbert E. Harris launches a campaign for the vacated seat.

In addition to her post on the Democratic National Committee, Duckworth has served as County Democratic chairwoman, and was chairwoman of the Virginia Delegation to the 1978 National Democratic Mid-Term Conference.

Pennino, who herself failed in congressional campaigns in 1972 and 1974, said Duckworth's free-wheeling style could well impede the freshman supervisor's dealings with the board. Pennino said Duckworth posed no threat to her own power on the board.

"Both Democrats and Republican people have ways of taking care of issues like this," Pennino said. "She just may find it very difficult to work with the other members of the board."

Fairfax Board Chairman John F. Herrity, a Rebublican, said yesterday that he had called Duckworth to congratulate her on her initiative.

"I say that if we can accomplish constructive steps in the interest of the people of this county by talking to the president, let's do it," Herrity said. p