In nominating federal appeals court Judge Douglas H. Ginsburg to the Supreme Court, President Reagan has turned from one of the country's best-known conservative jurists to one whose views outside the technical field of antitrust and regulatory law are largely a mystery.

In contrast to Robert H. Bork, Ginsburg, a soft-spoken and scholarly former Harvard Law School professor who has served on the court of appeals here for less than a year, has written little, if anything, on the contentious social issues he would have to grapple with as a justice, including abortion, civil rights, and separation of church and state.

Despite being relatively little known, his conservative backers in the administration pushed for Ginsburg above Judge Anthony M. Kennedy, the other leading candidate, largely because they believed he would be more "reliable," as one senior administration official put it, in adhering to conservative principles of judicial restraint.

Asked how the administration could be so certain Ginsburg would not turn out to be a surprise on the court, one senior Justice Department official said confidently, based on private conversations with Ginsburg, "He's one of us."

Ginsburg, said another department official, offers a "level of certainty, somebody that we all know, somebody that everybody is comfortable with."

If confirmed, Ginsburg, 41, would be the second-youngest Supreme Court justice this century, after the late William O. Douglas, who joined the court at age 40. His champions say that, despite his youth, his impressive resume -- law review editor, Supreme Court clerk, law school professor and senior government official -- illustrates his qualifications for the high court.

Four years ago, Ginsburg was an antitrust professor at Harvard Law School, unknown to Attorney General Edwin Meese III and other top administration officials who urged Reagan this week to nominate him.

Assistant Attorney General William Baxter, an admirer of Ginsburg's work on antitrust policy and regulation of financial markets, recruited the young scholar to join the Reagan administration.

"None of the political people at the White House had ever heard of him," said Christopher C. DeMuth, president of the American Enterprise Institute and Ginsburg's predecessor in a top job at the Office of Management and Budget, who recalled receiving phone calls asking who Ginsburg was.

But Ginsburg fit in comfortably with the administration and its free-market, deregulatory philosophy.

While still teaching at Harvard, Ginsburg argued at a debate on automobile safety regulations that regulations "stifled innovation," said Joan Claybrook, director of the National Highway Transportation Safety Administration in the Carter administration. "He judges whether or not standards ought to be issued for safety on the basis of their economic viability," said Claybrook, now president of Public Citizen, the lobbying group started by Ralph Nader.

Even as a young clerk fresh out of the University of Chicago Law School, Ginsburg was interested in the economic-based approach to law closely identified with the school, according to David Boyd, who clerked with Ginsburg for Judge Carl McGowan, a moderate on the appeals court here, and later on the Supreme Court, where Ginsburg clerked for liberal Justice Thurgood Marshall.

Despite the confidence of administration officials that Ginsburg shares their legal philosophy, Ginsburg's friends described him as a private man who rarely discusses political issues or offers his views on social issues.

"Doug's very professionally oriented," said Harvard Law School Professor Hal Scott, who has known Ginsburg since they attended high school together in Chicago, where Ginsburg's father owned a finance company. "I guarantee you, if anybody would know what his personal views would be, I would, and I don't know what they are. He's a private person. He keeps his views to himself. If we went out for a beer, we'd talk about banking regulation."

In his freshman or sophomore year at Cornell, Scott said, Ginsburg "was asking, 'What am I here to accomplish?'" and decided to take time off to figure out what he wanted to do.

He moved to Cambridge and met a man who had the idea of starting a computer dating service, and proposed to Ginsburg that he become a partner in what became "Operation Match."

"To him it was a business proposition," Scott said.

Scott said Ginsburg's approach to deciding cases is far different from Bork's.

"I see Bork as a person who staked out his position on a very theoretical level about his views of the Constitution from which he then deduced his attitude on all sorts of issues . . . . Ginsburg is completely different. Ginsburg is a guy who doesn't start with a theory. He doesn't try to pigeonhole it into a grand theory of anything."

Scott said that while Ginsburg "believes in the usefulness of economics to answer legal questions relating to business and regulation," he does not go as far as some law-and-economics adherents, such as 7th U.S. Circuit Court of Appeals Judge Richard A. Posner, in applying economic theories to decide other legal problems.

After a brief period as Baxter's deputy in the antitrust division, Ginsburg moved to OMB, where he succeeded DeMuth as administrator for information and regulatory affairs. At OMB, he implemented an executive order that expanded the cost-benefit test for reviewing proposed agency regulations to also determine whether the suggested rule was "consistent with the president's policies and priorities."

The executive order, according to Gary D. Bass, executive director of OMB Watch, a watchdog group often opposed to administration initiatives, "makes it a much more political decision. His contribution was to turn it more to the political arena."

In 1985 Ginsburg returned to the Justice Department to succeed Baxter as assistant attorney general in charge of the antitrust division. At his confirmation hearings, Sen. Edward M. Kennedy (D-Mass.) praised Ginsburg, saying, "He brings to this job a rare combination of scholarly background and academic achievement and accomplishment and administrative talent."

In explaining his philosophy, in an interview with The Washington Post, Ginsburg said antitrust law had been "perverted" at times in the past to prevent mergers that could have increased competition and bolstered the economy.

"In the 1960s, there were cases brought by the government to prevent mergers that in retrospect" were economically beneficial, he said in a 1985 interview.

"When one introduces noneconomic considerations to a statute that intended to operate on economic markets," he said, "that's intolerable, . . .unprincipled jurisprudence."

Ginsburg oversaw the initial stages of the Justice Department's attempt to remove the restrictions imposed on the "Baby Bells," the companies spun off from AT&T in a consent decree growing out of the department's antitrust suit against the telephone company. In several instances he overrode staff recommendations and declined to challenge corporate mergers.

After a year back at Justice, Ginsburg was tapped for a vacancy on the appeals court here. "I had talked to Doug before this all happened about whether he was interested in coming back to Harvard or going into private practice" after leaving government, Scott said. "This thing came out of the blue."

Since joining the court last November, Ginsburg has written 13 majority opinions on such issues as natural gas pricing, the impeachment of federal judges, the interpretation of the federal bankruptcy code and the constitutionality of the Samoan judicial system.

In a decision last month, Ginsburg rejected U.S. District Court Judge Alcee L. Hastings' challenge to a report by the U.S. Judicial Conference recommending that Congress consider impeaching Hastings.

Hastings, who was acquitted of bribery charges, contended that the judges' action violated the separation of powers. Ginsburg disagreed. "Congress simply provided statutory authority for the inherent right of the conference, if it so chooses, to advise the House that . . .it has come to believe that a judge may have crossed the line," he wrote.

Ginsburg sided with the Environmental Protection Agency in ruling that it had not delayed unreasonably in taking three years to issue a final rule on how the Clean Air Act applies to strip-mining operations.

In the 1987 edition of the Almanac of the Federal Judiciary, the space reserved for "lawyers' comments" offered this assessment of Ginsburg: "Too soon to say."

Indeed, even Ginsburg's friends find his sudden rise remarkable.

"Not in your wildest dreams do you go on the D.C. Circuit and then be picked for the Supreme Court within a year," Scott said.