Former presidential candidate Ronald Reagan, sounding very much as if he expects to make another run for the White House in 1980, said last night that President Carter's proposed defense budget cuts would weaken U.S. security in the face of "the new reality presented to us by the Russians."

Speaking to the fourth annual Conservative Political Action Conference on the eve of his 66th birthday. Reagan did what he had declined to do in Washington a month ago - criticize the new President directly and by name.

Former President Ford, whose last State of the Union message appealed for budget increases to meet the Soviet military buildup, has followed a policy of making no criticism of Carter until he has served several weeks, and perhaps months, in office.

Reagan limited his comments about Carter to national security, which Reagan called the "only major question on the agenda of national priorities." The former California governor opposed reductions in the defense budget, called maintenance of the U.S. commitment to "our Chinese friends on Taiwan" and opposed any U.S. negotiation over a change of status in the Panama Canal treaty.

As a candidate, Carter pledged never to relinquish "practical control" over the Panama Canal. Reagan said the President should refuse even to negotiate with the Panamanian regime, which he described as a dictatorship that had obliterated all civil rights. Sounding another familiar theme from his primary campaign, Reagan also defended the Central Intelligence Agency.

"Let's stop the sniping and the propaganda and the historical revisionism and let the CIA and other intelligence agencies do their job," Reagan said.

A less restrained criticism of Carter administration defense policies came from New Hampshire Gov. Meldrim Thomson, who said: "If the Carter administration is allowed to go unchallenged, and unchecked, on its present pro-Communist course, the day is close at hand when all that will be left for patriotic Americans will be to fight when there is no chance of victory."

Except for the comments about national security, Reagan's speech was a replica of one he gave in Washington last month before conservative academic group. Its central message was the conservatives should remain in the Republican Party and try to attract the support of "social conservatives among blue-collar workers and ethnics with Democratic affiliations.

One of the dogmas of the conservative movement is that a conservative majority already exists in the United States and is only waiting to be mobilized. Reagan cited Harris and Gallup polls showing that a majority of Americans regard themselves as conservative or right-of-center.

But the "conservative" means many things to many people and such a majority has been difficult to mobilize in practice except when the Democrats nominated George McGovern for President in 1972. Even then, many of the so-called social conservatives voted President Nixon and a Democratic Congress.

Reagan's speech last night indicated that he is aware of the many meanings of "conservative" and of the dangers of being "portrayed as ideological shock troops." He seemed to be gently urging his activist audience to broaden its own definition of conservatism and to stay within the Republican Party rather than create a rival political mechanism.

This course could have political value for Reagan if he decides to make another run at the White House. This week Reagan will launch a new political action group with nearly $1 million left over from his 1976 presidential campaign. It will be called "Citizens for the Republic," a name chosen to have the same initials as "Citizens for Reagan."

Reagan, the clear favorite of the 550 delegates attending the conference, was given a tumultous welcome when he was introduced late last night by Sen. Paul Laxalt (R-Nev.). Some members of the audience showed that the emotions of the 1976 GOP primary battle had not passed when they hissed pictures of Ford Nelson A. Rockefeller and House Minority Leader John J. Rhodes (R-Ariz.) during an eight-minute film clip of Reagan's performance at the Republican National Convention.

The only reservation expressed by any of the delegates about Reagan concerned his age.

"If he was four years younger, we'd be off and running right now," said James C. Roberts, executive director of the sponsoring American Conservative Union. "Reagan has both the emotional attachment and the affection of the people here. But there is a nagging feeling that he many be too old."

The conference also heard from another defeated favorite, former New York Sen. James L. Buckley.

Buckley gave a detailed and largely optimistic analysis of the status of conservatives, observing that the opposition to government regulation and centralization once celebrated by conservatives is now widely accepted by Americans. At the same time he warned that conservatives must talk about reforming government programs rather than eliminating them.

"In today's world, for example, it makes neither political nor moral sense to advocate a wholesale retrenchment in programs of public assistance in the expectation that private charities and families will immediately fill the void," Buckley said.

The conference concludes today with four conservative congressmen telling how they won election.

Buckley also favored the formation of a shadow cabinet by the Republican National Committee that would serve as a forum for commenting on Carter administration policies. In the parliamentary system of Great Britain, the "shadow cabinet" is the opposition ministry that is waiting to take over if the government falls.