Rex E. Lee, solicitor general of the United States, was excited and out of breath. That morning, Jan. 12, he had won his first big case representing the Reagan administration before the Supreme Court.

"It's a really important ruling," he kept saying to the reporter who had called on a different subject, "a really significant victory."

The justices, in a 5-to-4 opinion written by Justice William H. Rehnquist, had sharply reduced the ability of citizens' organizations to sue in the federal courts over government aid to religion and, in the process, had issued a ringing Reaganistic denunciation of judicial activism.

Citizens organizations do not have a "special license to roam the country in search of governmental wrongdoing and to reveal their discoveries in the federal court," Rehnquist wrote that morning. "The federal courts were simply not constituted as ombudsmen of the general welfare."

A month later, Lee was still talking about the decision, a "linchpin," he called it, of the administration's campaign against judicial activism.

There is no David A. Stockman of the war against judicial activism, but Lee is the closest facsimile. His office coordinates appellate litigation the way the Office of Management and Budget screens spending.

He believes too many people with too many causes have access to the federal courts. He believes too many "fundamental rights" have been created by the courts, and while he won't say whether the administration will try to eliminate some of them, he says it "will resist any opportunity to expand them."

The ideology of Rex Lee is laid out in his extensive writings, including his recent book, "A Lawyer Looks at the Constitution," which cites as examples of judicial excess the Supreme Court's rulings on school prayer, due process, equal protection and abortion.

The Supreme Court's 1973 ruling legalizing abortion, he writes, vested "in the judiciary the license to roam at will through the territory of legislative policymaking. If an unmentioned constitutional right can be pieced together by the judiciary out of bits and scraps that bear some resemblance to a variety of other provisions in the Constitution, then there is little limit to the extent to which judges can substitute their own judgment for that of the legislature."

The solicitor general is not in the business of making government policy, but it has been said that only a Supreme Court justice has greater power to influence constitutional and federal law. The government is the most frequent litigant in the Supreme Court, and Lee is the government's lawyer, with considerable authority and autonomy over the government's legal positions.

The solicitor general's office also has traditionally enjoyed clout and credibility with the Supreme Court, to the extent that one justice complained openly last year about his colleagues too often "mechanically" going in the direction urged by the solicitor general.

"The Office of the Solicitor General," Rehnquist wrote sarcastically in a dissent, "while having earned over the years a reputation for ability and expertise in presenting the government's claims to this court, is nonetheless a part of the Executive Branch, not of the Judicial Branch."

Lee presides over the solicitor general's office at a time of unusual tension. Some of the lawyers, considered among the best collections of legal talent in the country, worry about the credibility of the office with the Supreme Court because of signal-switching of recent months, particularly in the case involving tax exemptions for schools that discriminate.

Some are also worried about political pressure on the office, of the kind that produced its change in position in that case and attempted to influence its position in the Equal Rights Amendment recission-ratification case.

(Because of past involvements, Lee disqualified himself from both the ERA case and the tax exemption case.)

"The problem is that some people in this administration talk as if the Republican platform was adopted by referendum," said one lawyer in the office who asked not to be identified. "The fact that there was an election hasn't changed the U.S. Code."

But some of the more conservative administration members are upset that solicitor general's office hasn't done more to push Reaganism.

"They think they're running an independent satrapy over there," said one pro-Reagan lawyer in the department.

There have been conflicts with officials outside the office, for example, over the unwillingness of the solicitor general to intervene in a Mississippi case now before the court, on the side of those in favor of preserving sex-segregated colleges from constitutional attack.

Other complaints came from Lee's decision not to argue in a Connecticut case against a broad application of laws barring sex discrimination by institutions receiving federal funds. A change in that case would have been a change in government position.

"We've been asked to change position in many more cases," Lee conceded. "It is very proper for us to include implementation of an administration's policies as one of our overall objectives."

But he said he believes in "executive restraint" as well as "judicial restraint." And he recognizes that it doesn't look good to switch positions.

Rex E. Lee, 46, is only the second solicitor general in U.S. history from the West. The other was Charles Fahy, 1941-1945, from New Mexico. Lee grew up in northeastern Arizona, and did Mormon missionary work in Mexico. He graduated from the University of Chicago Law School, a center of legal conservatism, and clerked for Justice Byron R. White in 1963.

Lee was the first dean of the new Brigham Young University Law School, the Mormon law school, from 1971 to 1975. He interrupted his work there to head the Civil Division in the Justice Department in the Ford administration and then returned to Brigham Young until becoming solicitor general under Reagan last year.

His nomination as solicitor general probably would have gone largely unnoticed but for a "stop Rex Lee" movement organized by women's rights groups. They were upset about Lee's role as an adviser to the Mountain States Legal Foundation in the Idaho lawsuit against ERA ratification extension. And they were upset about another Lee book, "A Lawyer Looks at the Equal Rights Amendment."

The 141-page book, published by the Brigham Young University Press, was a compendium of women's rights decisions over the years, speculation about the ultimate legal impact of an ERA and a strong argument against its ratification.

National Organization for Women President Eleanor Smeal, testifying against his appointment, said that "at first glance his book appears to be a serious study . . . . But in fact, she said, "it is a mass of error, omission and distortion unworthy of a candidate for the position of solicitor general of the United States."

Lee defended the book as an "unemotional" and scholarly look at the ERA. "I am not the Phyllis Schlafly of the Rocky Mountains," Lee told The Washington Post in an interview during the controversy over the book last year. He was confirmed without serious opposition.

"The plea for judicial restraint, about which I feel very keenly, is not an absolute," he said in a more recent interview. "If it were, the courts would never do anything . . . . That would be a massive blow to the separation of powers."

But "any time that any court holds a statute unconstitutional, the necessary result is the substitution of one set of values for another.

"The courts are not in a good position to weigh those competing values. They are not roving commissioners . . . . That job is better done by the branch of government which is responsible," meaning the elected branches.

In the light of that language, Lee reacts uncomfortably when asked about the recent ERA ruling, in which Idaho U.S. District Court Judge Marion Callister swept away Congress' decision to extend ERA ratification. Callister's decision to hear the case in the first place are now being cited by liberals (and privately by some conservatives) as one of the most outrageous examples in the past decade of "judicial activism."

Is that a sign that conservatives are willing to urge activism on the courts when it works in their favor?

"I hope not," Lee said.