A Virginia State Senate panel today voted overwhelmingly to conduct a closed-door investigation of conflict-of-interest charges against Sen. Willard J. Moody (D-Portsmouth), a senior legislator who just last month was heading a conflict probe against another senator.

Both Moody and Sen. Nathan H. Miller (R-Harrisonburg), the subject of the earlier probe, will be given a "full, fair examination of any charges of conflict of interest" members of a bipartisan subcommittee of the Senate's Privileges and Elections Committee promised.

The resolution for the investigations, adopted 4 to 0 in a closed session, apparently marks the first time in almost a century that Virginia's legislature has initiated an investigation of alleged wrongdoing by any of its members.

"This is a serious step," said Sen. Joseph Gartlan (D-Fairfax). "It should not be interpreted as meaning that we have in any way prejudged the matter."

Moody, a Portsmouth lawyer, is one of the Senate's most senior members. He is chairman of the Senate Rules Committee, which recently began to investigate Miller on its own. "I welcome the opportunity for what I think will put the matter to rest, because it's been rehashed for years," he said today. "I think I do not have any conflict and I do not think the committee will feel I've had any."

Two years ago, Moody was publicly accused of conflict by another senator when he helped to kill a railroad caboose bill that had been opposed by the United Transportation Union. A major portion of Moody's legal clients are referred to him by the union, which represents railroad workers. In 1977, Moody also was accused by a Portsmouth lawyer of introducing a tour boat bill that was designed to assist a client who had lost a court case on the same question.

Republicans were critical of the bipartisan panel's decision to investigate Miller, a 38-year-old Harrisonburg attorney whose defeat at the polls last month was attributed in part to conflict questions that were raised after The Washington Post disclosed he voted and worked for passage of legislation that aided some of his law firm's clients.

"It's quite possible that this is window dressing," said GOP spokesman Neil Cotiaux. "They (the Senate Democratic majority) did not want to kick Nathan Miller in the teeth after his defeat, but neither did they want to exonerate him because that would upset their partisan cronies."

Miller was influential in pushing legislation that gave his legal clients, the state's electrical cooperatives, more than $13 million in tax breaks and business advantages. Miller has denied acting improperly.