The nationwide handgun control movement, after two years of gains following the shootings of President Reagan and John Lennon, has been thrown back on the defensive by the overwhelming defeat of a moderately worded gun control proposition that was on the ballot in California.

Its leaders are stunned by the extent of their loss: 63 percent to 37 percent. They blame their underestimation of the fear of crime and their opponents' 5-to-1 fund-raising edge.

"We failed totally in dealing with the fear of crime," said Michael Beard, director of the National Coalition to Ban Handguns. "The notion that owning a handgun is not related to your ability to protect yourself against crime seems obvious to us, but apparently it isn't to other people."

Beard's point is underscored by the county-by-county breakdown of the California returns.

The proponents of handgun controls had hoped to build up big margins in urban areas, where high crime rates have long produced a fertile ground for control legislation. But in the state's five most populous counties, the proposal, Proposition 15, lost 59 to 41. In the rural counties it was 3 to 1.

The opposition--spearheaded by the National Rifle Association and gun manufacturers--raised more than $6 million in the campaign against Proposition 15, which would have frozen the number of handguns in California at its current level, estimated at five million. Gun control proponents, on the other hand, raised just a little more than $1 million.

NRA officials contend that their paid media advantage was offset by pro-Proposition 15 editorials in 10 of the state's 11 largest papers and on most of the state's television stations.

"What beat them was voters, not money," said NRA spokeman John Aquilino.

This was a fight the gun control advocates initiated, with considerable fanfare and high hopes.

The timing, the forum, the ballot wording were all theirs. This was going to be the showcase campaign to demonstrate that while the politicians could be cowed by the strength of a single interest lobby like the NRA, the people couldn't.

The control proponents will continue their legislative fight in city and town halls, where they have made some progress in the past two years. In Washington, they will keep trying to block passage of a bill, McClure-Volkmer, that would gut much of the Gun Control Act of 1968.

Their new worry is that it will be more difficult now to make the case that gun control is opposed only by a narrow, committed segment of the electorate. And they have a further worry -- that, in the words of Beard, "Tom Bradley will become the new Joe Tydings around the necks of our movement."

He was referring to some post-election analyses of the California gubernatorial race that suggest the heavy turnout of rural, conservative voters spurred to the polls by Prop 15 proved costly to Bradley, who had supported the initiative. Tydings' loss of his Maryland Senate seat in 1970 has long been held out as a prime example of the political cost of supporting gun control.

Beard's group, which favors an outright handgun ban rather than halfway measures, deliberately stayed out of the California fight because the strategy of the Prop 15 campaign was to emphasize moderation.

To counter the NRA argument that the freeze was the first step toward confiscation or a ban, the initiative had a provision prohibiting the legislature from considering any future ban without voter approval. And to give it a tough-on-criminals edge, Prop 15 had several mandatory sentencing provisions.

The idea was to avoid the mistakes of Massachusetts in 1976 in the only other statewide referendum on gun control. It called for an outright ban, with the state buying back all privately held guns. It lost by more than 2 to 1.

Even though they had endorsements running the gamut from liberal financier Max Palevsky to Reagan "kitchen cabinet" member Justin Dart to industrialist Armand Hammer, the proponents raised just $1 million after completing their spring petition drive, instead of the $2 million to $4 million they had budgeted.

Charles Orasin, vice president of Handgun Control Inc., speculated that the nuclear freeze issue on the same ballot robbed Prop 15 of attention and liberal money.

Victor Palmieri, Prop 15 campaign director, said one problem was that no economic interest would benefit from the measure, while one, the gun makers, would be hurt. Joseph McNamara, police chief of San Diego and a leading Prop 15 advocate, said the measure was in the wrong state, one with a strong western, frontier heritage.

Beard and most gun controllers remain hopeful, though, that last week's setback will prove to be a "temporary glitch."

"I hate to say it, but one well-publicized homicide could wipe that California vote out," he said.