Virginia's Republican lieutenant governor candidate, Nathan H. Miller, last year introduced and helped secure passage of a state law designed to help one of his law firm's longstanding clients win potentially lucrative mineral rights.

After a state judge ruled Miller's law unconstitutional and threw out a suit brought by two of Miller's law partners on behalf of the client, the Harrisonburg state senator this year got a fellow legislator to offer legislation that allowed the suit to be resurrected. Both bills passed without dissent, and Miller was among those voting in their favor. The suit is still pending.

Miller refused yesterday to say whether he believed his actions constituted a conflict of interest. "You're asking me to make a judgment I can't make," he said. "I have commented on the facts. That's the best I can recollect, and I really don't have any further comment on it."

Miller, who in 1976 won his state Senate seat after accusing his opponent of conflict of interest, acknowledged yesterday that he introduced the legislation because the client "had indicated he had a problem." Miller said, however, that he had no idea that two of his law partners, one of whom is his brother-in-law, would file a lawsuit based on the legislation he introduced.

Miller's legislative efforts in the mineral rights case, which included Miller's committee testimony in favor of the change, mark the second disclosure during the campaign of how Miller has used the power of his elected office to benefit those who have paid fees to his law firm.

Two months ago, The Washington Post reported that Miller's firm accepted at least $250,000 in legal fees from the state's electrical cooperatives while Miller was drafting and voting for legislation that gave the co-ops $13.2 million in tax breaks and business advantages. Miller's Democratic opponent, former Portsmouth mayor Richard J. Davis, has charged that Miller's actions amounted to a violation of the state's conflict-of-interest law and virtually all of the state's major newspaper editorial writers have criticized Miller's involvement with the co-ops.

Miller then defended his actions by saying that he wears "two hats" in the legislature -- one as a senator and one as a private lawyer. If the citizens of Virginia want a part-time legislature, he said, "then there is going to be the necessity of their citizen legislators to make a living."

Virginia law does not expressly prohibit lawyer-legislators like Miller from voting on bills that may affect them or their clients. The rules of the Senate, however, bar members from voting on matters in which they have an "immediate, private or personal interest."

But partisan Democratic response was swift yesterday. "I think the time has come that this thing ought to be investigated," said Sen. Adelard L. Brault (D-Fairfax), one of the Senate's leading advocates of stricter ethics laws. "It's a little bit shocking and I think we ought to do something about it."

Miller's firm has made less than $5,000 on the mineral rights matter and will not gain financially if oil or natural gas is found on its client's land, said a source within the firm. The client is a part-owner of Riddleberger Brothers Inc., one of Harrisonburg's largest contracting firms. The company has for more than 20 years paid the firm of Litten, Sipe and Miller a set annual fee plus hourly charges for its services. A firm official declined to give the amount of that retainer.

H.D. (Ikey) Riddleberger Jr., for whom the legislation was introduced, yesterday said Miller, 38, had done "very well" in seeing that the legislation passed. "Since they are my attorneys, I would think he Miller would do what I asked him to if I pay him," Riddleberger said.

Riddleberger is seeking to gain the mineral rights to 220 acres of land he owns with his wife in Augusta County, a western Virginia county outside Miller's district, and nestled in the foothills of the Alleghany Mountains. He had been approached by several firms eager to lease the rights for oil and gas drilling, he said.

The mineral rights on Riddleberger's property had been held by Chesapeake Western Railway, a subsidiary of Norfolk & Western Railway Co., for more than 35 years, although the railway had never explored the mineral content of the property.

Riddleberger said his attorney, Miller's brother-in-law and law partner James V. Lane, told him there was a state law designed to help landowners win back mineral rights unclaimed for 35 years or more, but that the law did not apply in Augusta County.

In 1980, the following year, Miller introduced a bill that would have made it easier for landowners to prove their legal claims to mineral rights in all counties in the state. When Blaine Carter, president of the Virginia Coal Association, objected, arguing that such a law would jeopardize some coal company mineral holdings, Miller altered his bill to affect only Augusta County. He voted with the rest of the Senate 40 to 0 to approve it. Two days after Miller's bill became law, his senior partner, Donald D. Litten, and his brother-in-law filed suit on behalf of Riddleberger and his wife, Barbara.

Augusta County Circuit Judge William S. Moffett Jr. dismissed the Riddlebergers' suit, however, holding that Miller's law was unconstitutional because it was drawn too narrowly. Miller, who was campaigning for the GOP nomination for lieutenant governor, went back to the General Assembly within weeks and asked Sen. Daniel W. Bird Jr., (D-Wytheville), who was introducing mineral rights legislation for two other counties, to introduce an amendment designed to overturn Moffett's ruling.

"At no time did Nathan divulge to me that his law firm was affected, or had a case pending before the court," Bird said. "He just said that a judge had ruled that our law from the previous year was unconstitutional."

Miller said yesterday that he couldn't remember whether or not he told Bird of his firm's involvement in the Riddleberger case. "I don't recall whether I did or I didn't," he said. "Should I have? The case was over."

Bird said the amendment he proposed at Miller's request broadened Miller's 1980 law by extending the law's coverage to all counties within a certain population range. The population range Miller suggested to Bird was a narrow one similar to previous mineral rights amendments -- from 53,000 to 54,500 persons. It applied to only one county in Virginia: Augusta County.

"It did strike me that it was unusual that he wanted to amend the bill to cover Augusta County," because Miller doesn't represent the area, Bird said. "But I said if it was all right with Sen. Frank Nolen a Democrat who represents Augusta County , I wouldn't fuss yea or nea."

During the time that Miller's amendment was going through the legislature this spring, his brother-in-law, Lane, was lobbying in its behalf. Lane said yesterday he sent letters of support for the Bird bill to key committees and members of the General Assembly. Lane said he didn't write the letters on his legal firm's stationary, and he said he did not mention his association with Miller or his law firm's involvement in the Riddleberger case.

Bird's bill was approved by the Senate on a vote of 37 to 0, with Miller voting "aye." Miller's law partners brought suit for Riddleberger against the Chesapeake Western Railway under the new law the day after it took effect.

Ronald D. Hodges, an attorney for Chesapeake Western, argued in papers filed in Augusta County Circuit Court that the 1981 amendment was "special, local and private legislation" and was unconstitutional.