The Reagan administration plans to nominate two prominent conservatives -- former Sen. James L. Buckley and Office of Management and Budget general counsel Michael J. Horowitz -- to the U.S. Court of Appeals here, sources said yesterday.

Those nominations, along with another to the same court pending before the Senate, would end more than two decades of liberal domination of a court often described as the most powerful in the country after the Supreme Court. Its jurisdiction is broader than that of any other appeals court in the nation over civil rights, environmental and regulatory controversies.

The expected appointments are part of the administration's overall effort to secure a conservative federal judiciary throughout the country long after President Reagan leaves office.

Reagan already has picked three members of the 12-member court. Buckley and Horowitz, if formally nominated and confirmed, along with recently nominated Laurence H. Silberman, would give the administration six solid votes. There are five generally liberal votes on the court, with Judge Ruth Bader Ginsburg considered a swing vote.

Buckley, 62, a former Conservative Party senator from New York and more recently head of Radio Free Europe/Radio Liberty, had been considered for a vacancy on the 2nd U.S. Circuit Court of Appeals, which covers New York, Connecticut and Vermont.

Sources said strong opposition from liberal Republican Sen. Lowell P. Weicker Jr. (Conn.) and several other factors eventually convinced the administration to nominate Buckley for the D.C. court instead.

Horowitz, 47, is a leading administration neoconservative noted for his efforts to limit the use of federal funds for political advocacy by nonprofit groups, contractors and others who receive federal money. Those efforts upset both liberal organizations, who accused him of trying to "defund" their activities, and corporate recipients of federal funds. He also has been active in the administration's attempt to cut funding for the Legal Services Corp. and has spearheaded a move to limit the amount of attorneys fees available to civil rights lawyers under federal law.

Horowitz, a 1964 Yale law school graduate who taught civil rights law in Mississippi during the 1960s, spent 15 years in private practice, largely in his own, single-lawyer firm here and in New York before former OMB Director David Stockman asked him to join the Reagan administration in 1980.

Buckley's odyssey from the New York-based court to the D.C. court mirrors that of current D.C. appeals court Judge Kenneth W. Starr two years ago. Starr was first considered for the Richmond-based appeals court, but encountered opposition from Sen. John Warner (R-Va.) and other Virginia GOP leaders.

The administration then decided to put him instead on the D.C. circuit, where no home-state senators could oppose him.

In Buckley's case, the administration picked him about six weeks ago for a vacancy on the New York-based court. When notified of the move, Weicker, who once said Buckley "represents everything I have fought against in the Republican party," immediately opposed the nomination but said he would not fight it publicly at least until after the background clearance process was completed, sources said.

Administration officials considered having Sen. Alfonse D'Amato (R-N.Y.) sponsor Buckley as a New York candidate. But Buckley, widely criticized for moving to New York to run for the Senate in 1970, said he now lived and voted in Connecticut and would not agree to be sponsored by a New York senator, sources said.

On Sept. 20, the New York City Bar Association, in an unusual move, publicly announced that it would not approve Buckley, who had practiced law for only a few years, for the court. Buckley, on the advice of the Justice Department, had refused to meet with the city bar association.

The New York City assessment, though not binding on the full American Bar Association, nevertheless troubled administration officials.

The administration had been slow to fill two longstanding vacancies on the D.C. court. Silberman, a former deputy attorney general, was finally picked after a lengthy internal review, but local attorney Marion Edwin Harrison was not selected after a similarly long period.

The death of appeals court judge Edward A. Tamm in September created a third vacancy on the court, increasing the urgency to fill the vacancies, sources said.

Administration officials felt Buckley would be more familiar with the legal issues faced by the D.C. court, sources said, and that the appointment here would not place Weicker in a difficult political situation. A tentative decision to switch Buckley came last week and was made final yesterday, sources said.

Patrick B. McGuigan, of the conservative Free Congress Foundation, yesterday hailed the prospect of the two nominations. "Just fantastic. . . . We're going to take control of the most crucial civil rights court."

"These nominations would make the court an activist and radically conservative judiciary," said Nan Aron, who heads a coalition of liberal groups, the Alliance for Justice. "Horowitz's nomination will be a lightning rod for opposition" by civil rights and other organizations, she said.