Six months ago, morale among Britain's antinuclear activists was as high as it had ever been.

The Greenham Common women had scored a bulls-eye in attracting attention at home and abroad to their vigil at the gates of the air base where U.S. cruise missiles are to be deployed.

Opinion polls showed a majority of the British people opposed to the missile siting. The country's second largest political party, Labor, pressed its commitment to unilaterally ending Britain's nuclear role. Prime Minister Margaret Thatcher, embarking on an election year, called for greater flexibility by the Reagan administration toward the Soviets in its arms bargaining stance.

Today, much of that momentum has been spent. The Greenham Common protest has gone stale, antinuclear activists acknowledge. Labor's policies were overwhelmingly rejected in last week's general election. The country's leading pollster now finds a majority of its sample backing the cruise deployment.

Thatcher has embraced Reagan's new nuclear negotiating strategy in every detail. Vice President Bush encountered no significant antinuclear protest in Britain last week, in contrast to the violent demonstrations that met him in West Germany a day later.

Accurately grasping the scale of the problem, a supporter slipped a note to Msgr. Bruce Kent, general secretary of the Campaign for Nuclear Disarmament, at a church-sponsored rally the other night. "For God's sake," it said, "don't run out of steam yet."

As Britain wearily emerges from the political excitement of the election period, the antinuclear movement is searching for a way to recover some of the ground it has plainly lost. Demonstrations are planned through the summer and fall and some may be spectacular enough (a human chain across Hyde Park from the Soviet Embassy to the U.S. Embassy on July 16, for instance) to revive flagging spirits.

But leaders recognize that public sentiment has become largely inured to protests and that the challenge of forcing meaningful changes in Britain's defense policies, let alone that of the superpowers, is formidable. "We have always known," Joan Ruddock, chairwoman of the Campaign for Nuclear Disarmament, said in a speech last weekend, "that if the political process failed to respond to . . . opposition on cruise we could not physically stop their deployment."

The time had come, she said, for a renewal of energy in mounting nonviolent direct action--blockades at bases and similar confrontations designed to produce arrests for civil disobedience, pickets at Parliament and the lobbying of legislators.

But when Rebecca Johnson, a stalwart in the Greenham Common group, ran for Parliament against Defense Secretary Michael Heseltine, she received only 517 votes out of 45,309 cast.

While more than 200 members of Parliament got elected on Labor's platform of nuclear disarmament, no evidence was found that the nuclear issue was decisive in their success. In fact, public opinion appears to be moving in the other direction, at least on cruise deployment.

In January, Market and Opinion Research International reported that 54 percent of British people polled opposed cruise. Its survey in late May found that 52 percent favored the impending deployments and 34 percent were opposed.

But to such key figures in the antinuclear movement as Kent, understanding what has happened is crucial to recapturing lost supporters.

One cause, he said in an interview, has been errors by well-meaning local organizers. In Upper Heyford, site of a major U.S. air base, protesters pledged to "close" the base for several days in early May just as the election campaign began.

Although demonstrators succeeded over three days in disrupting access to the base for brief periods, police quickly removed them, making 750 arrests. The media portrayed the effort as a failure, said Kent, because the base had not been closed down and, practically speaking, never could be.

"After all, they have tanks they could use if they needed to," Kent said. "We have to be careful not to make promises we can't keep."

The image of the Greenham Common women has also suffered at the hands of the critical media, with the protesters increasingly identified with radical feminism.

But perhaps the most important reason for the antinuclear movement's problems, according to Kent and others, is that its goals have become confused in the public mind. Instead of focusing on specific targets such as cruise or Britain's purchase of U.S.-made submarine-launched Trident missiles, the campaign has been shifted to the general question of Britain's nuclear policy.

During the campaign, Thatcher and Heseltine concentrated on presenting any opposition to nuclear policy as backing for what they called "one-sided disarmament."

"While our aim of making nuclear weapons a major focus of the election was achieved," Ruddock said last weekend, "our opponents turned it to their own advantage."