Virginia Senate leaders today called an extraordinary session of the Senate's rules committee for Friday as the first step in determining whether state Sen. Nathan H. Miller, the Republican candidate for lieutenant governor, should be disciplined for conflict of interest.

The committee meeting will mark the first time in more than 100 years that a Virginia legislative panel has even discussed investigating a fellow legislator. No one has been expelled from either house of the General Assembly since 1875.

The announcement by the legislature's Democratic leaders, which GOP campaign aides immediately branded as partisan politics, followed disclosure that Miller introduced and help secure passage of state laws that could assist one of his law firm's longstanding clients gain control of potentially lucrative mineral rights. It was the second round in two months of conflict allegations against Miller, who also drafted and voted for bills giving his electrical cooperative law clients millions of dollars in tax breaks and business advantages.

Miller today issued a five-page written statement denying he had a conflict on the mineral rights law and asserting that a Washington Post account of his role in the matter was distorted. Miller's campaign manager, Joseph Loyacano, said the article contained "out and out lies," but disputed few of its details.

"I am not angry. I am heartsick," Miller told newsmen at his campaign headquarters here. "I feel in my heart there is no conflict of interest in fighting for legislation that benefits all people," he said.

Interviewed later as he entered a $1,000-a-couple reception in Norfolk tonight, the candidate appeared dejected. Asked if he had introduced any other legislation on behalf of his law firm's clients, Miller said "I don't know . . . I've been in the legislature 10 years."

The state senator, who was publicly supported by Republican officials after the first conflict disclosure, virtually was alone in his own defense today. Republican Gov. John N. Dalton declined to defend Miller's conduct. "You'll have to let Nathan Miller answer his own questions as far as that's concerned," the governor said.

Miller's running mate, gubernatorial candidate J. Marshall Coleman, sidestepped questions. "I have confidence in the integrity of my running mate," he said in a Roanoke campaign stop and declined further comment.

While most Republican campaign officials remained silent, several said privately they disapproved of Miller's actions but could not afford the appearance of dissension two weeks before the most hotly contested state election in recent memory.

Miller, a Harrisonburg attorney, contended in his statement that he introduced a bill in 1980 to benefit not only H.D. Riddleberger Jr., his law firm's client, but also "numerous farmers, some of them clients and some constituents." They all sought the law so they could sell mineral rights to their lands, Miller said.

Miller's bill, which was drawn to cover the entire state, passed after Miller agreed to restrict it to Augusta County, site of Riddleberger's property. The county is outside Miller's senatorial district. The measure, which cleared the senate without dissent, was declared unconstitutional by a state judge after Miller's law firm filed suit for the Riddlebergers.

At the 1981 session, Miller persuaded another senator to broaden the mineral rights law's reference to Augusta and add Rockingham County, which is in his district. That bill also passed with Miller voting aye and was signed into law. His law firm has brought a new lawsuit under that law on behalf of the Riddlebergers.

"I am heartsick that my motives have been construed in such a fashion, saddened that my efforts to eliminate discrimination against my district has been made to look like some attempt at personal gain," Miller said. "It seems to me that my motives, and mine alone, are being subjected to microscopic examination on every bill I have ever introduced, regardless of its merits and regardless of its needs."

A spokesman for Richard G. Davis, Miller's Democratic opponent, today accused the GOP nominee of evading the issue. "I just don't see that anything in here addresses the real issue," said Joe Lockhart, Davis's press secretary. "The real issue is: Did he use his public office to benefit his client and himself?"

Miller's law partner, James V. Lane, told The Post he had lobbied for the bill by writing letters to lawmakers. Loyacano today said no such letters had been written.

Loyacano said Friday's rules committee session was inspired by electoral politics. "This is . . . a Democrat-Republican issue." He also noted new radio ads by Miller's Democratic opponent, Richard J. Davis, attacking the Republican's alleged conflict on co-op bills began running today. "It's a heck of a coincidence," the Miller aide said.

Conflict-of-interest charges against legislators have been widespread in recent years but none has led to a legislative hearing. Rules committee chairman Willard V. Moody was accused in 1979 by a fellow senator of conflict after Moody helped scuttle a bill opposed by the United Transportation Union, one of the biggest clients of Moody's Portsmouth law firm.

One Democratic state senator said today that Moody investigating conflict-of-interest charges is "like having Jimmy Hoffa investigate an irregularity in a union election."

Senate rules require members with "an immediate, private or personal interest" in a bill to abstain from voting. Such abstentions traditionally have been left to the discretion of individual senators under the concept articulated by Senate Majority Leader Hunter B. Andrews. "There is an honor code that all of us who went to school in Virginia are familiar with," he has said.

Under current rules the assembly may expell its members for serious violations.

Miller's latest dilemma surfaced on the same day that his electric cooperative clients announced an agreement in principle to purchase portions of a power plant from Virginia Electric and Power Co. Under legislation drafted by Miller and passed by the General Assembly last year, the co-ops should save approximately $10 million on the purchase.