he nuclear freeze started out as the glamour girl of the California election. In the state that likes to be out front, where the pursuit of happiness and even peace seems plausible, this of all issues and candidates seemed the one most clearly destined for success.

But today the nuclear freeze is suffering from some wasting disease. The rosy tan has faded, its almost 2-to-1 margin has shrunk to a mere 6 points.

Citizens of the with-it state lagging behind conservative New England town meetings? Not all Californians are baffled by this dramatic decline. Harold Willens, the indefatigable millionaire peacenik who runs the freeze campaign, says what happened to it is "a covert campaign financed by the taxpayers."

What he means is that despite a public posture of permissiveness bordering on indifference, the administration has waged a vigorous counteroffensive, swamping the state with high-level spokesmen, not to mention providing lesser fry from the State and Defense departments for talk shows and roundtables.

It was a point of pride with the Reagan White House to dampen a huge vote of no confidence in the president's disarmament policies in his home state. The commander in chief himself led off last July in Los Angeles with a statement that the freeze "would make this country desperately vulnerable to nuclear blackmail." Since then, of course, he has said its sponsors seek "the weakening of America."

Defense Secretary Caspar W. Weinberger, U.S. Arms Control and Disarmament Agency Director Eugene V. Rostow, Navy Secretary John F. Lehman Jr., and Lehman's brothers -- one from ACDA, the other from State -- have all been through, raising the alarm about the dangers of halting the nuclear arms race.

The freezers were initially put off by the total absence of the usual high-priced, high-visibility campaign associated with the White House. Two modest committees, Californians for a Strong America and the Committee for Survivable Reductions, were formed but made little noise.

Freeze advocates began to suspect what they were up against when Willens, scheduled to debate conservative Rep. Robert K. Dornan (R-Calif.) before the San Francisco World Affairs Council, found himself confronted at the last minute by Judith Mandel of the State Department's strategic nuclear planning bureau.

Rear Adm. Eugene J. Carroll Jr., one of several freeze advocates among the retired military, began to think he was being followed. "Every time I went on a talk show, somebody from State or Defense showed up, demanding equal time," he said.

The supreme substitution occurred last Friday when national security adviser William P. Clark appeared at the Hilton Hotel, pinch-hitting for his deputy, Robert C. McFarlane, who had been offered to the San Diego City Club and Chamber of Commerce. Four days before the meeting, Clark volunteered to come himself and to give what he called "a major foreign policy speech."

He gave instead what sounded like a major last-minute pitch against the freeze. He told of a cable from arms negotiator Paul Nitze, who reported from Geneva about Soviet "smirking" on "hearing of the success of the freeze movement and the simplistic way that some church groups are approaching this particular problem."

Clark compared the president to his grandfather, the sheriff of Ventura County, who kept a Colt .45 named a "Peacemaker" on the wall "and never used it against anybody."

The State Department does not care to discuss its role in freeze activity in California or the other eight states where the freeze is on the ballot. But Thomas Bleha, director of the public programs office, acknowledged to the St. Louis Post-Dispatch that three-fourths of the office's activities had occurred in freeze states and that almost half had been initiated by the department. State sent 17 representatives to California for a total of 57 appearances.

According to department spokesman Alan Romberg, "It was a pretty standard thing, as on any major foreign policy issue."

These efforts were abetted by heavy mailings from right-wing groups and frequent warnings from nuclear experts of the Lawrence Livermore Laboratory about how much of California's economic health depends on defense manufacture.

Local considerations have figured. Proposition 15, a stiff gun control law on the ballot, has galvanized the state's shooters, who are not receptive to disarmament on any level. Gov. Edmund G. (Jerry) Brown Jr.'s inflammatory pro-freeze commercials, which had to be yanked, did not help.

Contemplating a suddenly shy California, Randy Forsberg, a founder of the national freeze movement, said, "When challenged by expert opinion, people retreat. They think there is something they don't know."