In her 17 years in the Virginia General Assembly, Mary Aydelotte Marshall hasn't built a reputation as one of the legislature's street fighters.

This year, however, "the gentlewoman from Arlington," as her 99 fellow state delegates call her, has shown that she can be as sharp and crusty as any of the men who traditionally have dominated the Virginia General Assembly.

Marshall, 64, a Democrat, has emerged as a leading critic of what some say is a concerted move to weaken the state's conflict-of-interest law.

"I didn't come down here to do that . . . but somebody had to speak out. It's the most important issue," said Marshall.

"Corrupt government is very hard to get rid of once you start down that path," she said in an interview in her office overlooking Capitol Square. She piqued the ire of some lawmakers here and in Annapolis recently when she suggested that the Virginia lawmakers were in danger of "appearing to tolerate sleaze" much like their counterparts in Maryland.

In the past such allegations were viewed by leaders of the Virginia assembly as another example of the rumblings from what some called "the People's Republic of Arlington" and one of the most liberal delegations in the House.

Her comments on the ethics issue this year are said to have irritated some of the House's leaders, but this year legislators are finding that they need to take Marshall more seriously.

Anyone who underestimates Marshall, said Del. Dorothy S. McDiarmid (D-Fairfax), "will find she probably knows twice as much as they do." McDiarmid, chairman of the Appropriations Committee, said Marshall "relates very well . . . . She has an absolutely marvelous sense of humor and will come up with a wonderful one-liner . . . that can always break the tension."

Thanks to her seniority, Marshall this year became chairman of a major committee -- Cities, Counties and Towns -- that reviews legislation affecting local governments. Her continued victories in Arlington also have made her a ranking member of the powerful Privileges and Elections Committee, which handled the ethics issue; of the Committee on Health, Institutions and Welfare, where she is a recognized expert on the elderly and mentally ill, and of the Roads and Internal Navigation Committee, which handled seat belt legislation this year.

"You somehow gain in importance; your advice and support are sought," said Marshall, acknowledging that her new position has increased her influence but playing down the importance of her cities panel. "It's a minor committee of the major committees," she said.

"Mary has always been one for openness in general," said Del. J. Samuel Glasscock (D-Suffolk). Glasscock said he thought that Marshall's "sleaze" remark was "pretty strong," but he said the issue is not new to her. "She feels very strongly that we ought to be open and accountable, and she is right about that."

Marshall was unable to sway the House from weakening the conflicts law, but her views are likely to be supported more strongly in the Senate where she expects a tougher law to emerge. Senators, she said, are more embarrassed by the conflict-of-interest criminal charges facing Sen. Peter K. Babalas (D-Norfolk), the chamber's fourth-ranking member, than are delegates.

At the heart of the debate is how much leeway Virginia's 140 legislators, who serve part time, should have in voting on issues that affect their livelihoods. "I recognize all the problems, that people have to make a living," said Marshall, a Phi Beta Kappa graduate of Swarthmore who chose after World War II to give up her career as a government analyst to become a full-time housewife, civic activist and mother of three.

In Richmond, Marshall long has been a leading proponent of open Democratic caucus meetings, regularly forcing secrecy-minded delegates to raise their hands publicly in votes to bar reporters and others. The meetings still get closed, but "the number of hands gets fewer every year."

Marshall has won praise from colleagues and some editorial pages, but she said she has irritated key House leaders, including Speaker A.L. Philpott (D-Henry) and Del. Theodore V. Morrison Jr. (D-Newport News), chief architects of the House ethics proposal.

Morrison "doesn't like Northern Virginia . . . " said Marshall, who once flaunted her suburban Washington base with a bumper sticker proclaiming: "G. Washington was a Northern Virginian."

Marshall's political roots in Arlington date to her efforts to end Massive Resistance to school desegregation in the late 1950s. She won her first election in 1967, but she lost, as did many other Democrats, in 1969 after President Nixon aired commercials calling for a Republican vote. Marshall and the Democrats regrouped to win again in 1971. "There's a myth that I'm the only Democrat who can carry my district. While it's not true, it adds to my electability," said Marshall, who leads the ticket each election year.

Her problems in Richmond are quite different. "I always have trouble with public relations because my bills are so arcane," Marshall said. Bills this year include measures to define "ambulatory and nonambulatory" medical care, to require X-ray inspections and to allow dental hygenists to work "under the direction of" rather than "under direct supervision of" dentists.

Marshall keeps a low profile at night, shunning the big parties here that provide prime-time entertainment for most legislators. "I go out to dinner with friends and then fade," she said.

Her husband Roger, a businessman, rarely leaves their North Wakefield Street home to go to Richmond, and he plays only a background role in her campaigns. "He goes to all the parties he thinks he'll enjoy," Marshall said, suggesting that the list is short.