Democratic gubernatorial candidate Charles S. Robb said yesterday it was "indefensible" that Nathan Miller, Republican candidate for lieutenant governor in Virginia, helped secure passage of legislation that could benefit his law firm's clients.

But Robb said any investigation of Miller's conduct by the Democratic-controlled legislature should be delayed until after the Nov. 3 election "so that elements of partisanship don't cloud the merits." The state Senate Rules Committee is set to discuss Miller's action in a special meeting today. The committee will discuss Washington Post reports that on two occasions Miller drafted and voted for legislation that would give financial advantages to his law firm's clients.

"There are a number of elements that trouble me," Robb said while campaigning yesterday in Mount Vernon. "But more than anything else, it is that no disclosure was made."

Robb's opponent, GOP gubernatorial candidate J. Marshall Coleman, speaking in nearby Springfield yesterday afternoon, declined to defend his running mate, even after a meeting with Miller in which "he told me his version of it." Coleman said the issue is "factually complex and in dispute . . . I'm not sufficiently conversant to make a judgment." Coleman refused to discuss the report.

What the General Assembly should do, Coleman said, is begin "a very much needed and long overdue" revision of the conflict-of-interest statute governing official conduct. Coleman was echoing a suggestion made earlier this month by his campaign chairman and mentor, Gov. John N. Dalton, in relation to still another conflict-of-interest report involving a member of the state Highway Commission.

Coleman said any review should be conducted "in a nonpartisan, bipartisan format," and "ought not to single out any one person . . . It would be helpful to form a commission to make a judgment" about what constitutes conflict of interest.

Other Republican Party leaders joined in mounting an attack on the proposed investigation. State GOP Chairman Alfred Cramer III called today's scheduled meeting the "convening of a kangaroo court to conduct a witch hunt." He said the public should ask why, "after turning its back on serious charges of conflict of interest raised against Democrats during the last several years, just 12 days before the election Democrats are dusting off the conflict-of-interest law . . . for the first time in 100 years."

Former governor Mills E. Godwin also came to Miller's defense, releasing a statement: "I am voting for Nathan Miller. I know him to be a gentleman of ability and a man of honor and integrity."

Assistant Senate Minority Leader Wiley F. Mitchell Jr. of Alexandria, one of Miller's top campaign advisers, agreed with Coleman that any probe "should focus on what the conflict-of-interest rule means and how it applies."

Mitchell said "the question of whether he Miller exercised good judgment is up to the public to decide." Mitchell added, "If I had been in his position, I would not have voted" on the legislation.

Mitchell said there are many potential conflicts of interest for citizen-legislators, and that there are particular problems for lawyer-members such as Miller and himself. "I have disqualified myself when I was not sure disqualification was proper to my constituents."

Earlier this year, The Washington Post disclosed that Miller had drafted and voted for bills giving his electrical cooperative clients millions of dollars in tax breaks and business advantages. The Post reported Wednesday that Miller introduced and helped secure passage of legislation that would help another client of his law firm gain control of potentially lucrative mineral rights.

"I feel in my heart there is no conflict of interest in fighting for legislation that benefits all people," Miller said in response to the new report.

Mitchell said it has been common practice for legislators with ownership interest in banks or who serve on bank boards, to vote on banking legislation. "It's not a defense," Mitchell said, "but it happens . . . the very same conduct has been condoned in the past without any attempt to call it to task . . . but because of press disclosures it's not to be tolerated any more."

Mitchell said there is "a desperate need for a nonpartisan body to determine how the rule should apply." He said the Senate Rules Committee should not conduct the investigation because its membership is disproportionately (14-1) Democratic.

Meanwhile, Common Cause of Virginia responded to an earlier call from Dalton for a weakening of the existing conflict-of-interest law: "Quite apart from being too detailed, it is not inclusive enough." Common Cause said the present act "mainly requires disclosure -- that is certainly not too much to ask of those who run the state, those trusted by the public to conduct the government's business."

And at a press conference in Richmond, Common Cause state Chairman Jane Reuss charged that Robb and Coleman were operating "smear campaigns" and had violated pledges to avoid mudslinging. She denounced both campaigns as "an insult to the intelligence of Virginia voters," and said the election "could go down in history as one of the least informative, most vitriolic gubernatorial races in Virginia in this century."

Reuss was particularly critical of radio and television advertising from both candidates on drug abuse. She accused both sides of "gross distortions." Reuss said the "gutter tactics" concealed the fact that there are real "differences in temperament, philosophy and outlook" between the two candidates.