Presidential candidate Edward M. Kennedy almost took the full-scale antinuclear pledge today, but then settled back to a position that leaves him more opposed to nuclear power than President Carter, but slightly less so than California Gov. Edmund G. (Jerry) Brown Jr.

Kennedy, who seemed weary and distracted today after a new poll showed him losing ground to Carter among Democrats nationwide, gave a somewhat muddled answer in an exchange with an antinuclear activist that seemed to suggest he favors a permanent ban on licensing new nuclear plants for generating electricity.

However, Kennedy's staff explained later that the candidate is for a two-year moratorium, but not a final bar, against the new licenses.

Meantime, Vice President Mondale, in the state for a brief campaign swing, took a notably soft line toward Kennedy, declining to express any criticism of the senator's recent roundhouse assaults on administration policy. c

"The American people are not interested in negative campaigning," Mondale told a Manchester press conference, apparently accepting the verdict of recent polls that Kennedy has hurt his own campaign by his drumbeat of criticism of Carter's domestic and international policies.

Mondale, who insisted that the New Hampshire contest is "very very close," despite the polls, contented himself with the assertion that the "steady, consistent, realistic policies" of the administration "will work," if Carter is given a second term.

Kennedy has been battling with Brown for the support of New Hampshire's active antinuclear community, and he scheduled a major speech today to emphasize his credentials in that area. His address called nuclear power "an idea whose time has passed" and demanded that the government phase out all current nuclear reactors as soon as alternative energy sources are found.

But a phase-out of existing plants is only half of what environmental groups consider a "pure antinuclear position. The purest, including Brown, also say that work should be stopped for good on the 92 nuclear generating stations now under constuction.

Thus, when Kennedy finished his prepared speech today, a member of the Campaign for Safe Energy, an antinuclear group, stood up and asked, in polite but pointed tones, "Would you stop construction and deny licenses to the 90 plants that are now being built?" Kennedy hemmed and hawed for a few seconds and then replied, "I would have a moratorium -- the answer would be, is, yes I would."

This seemed to suggest that Kennedy had embraced the complete anti-nuclear position -- which would have been a tougher stance than he had taken before. Previously, he has said only that plants that are less than 40 percent completed should be converted to nonnuclear energy sources.

Harvey Wasserman, a Clean Energy Campaign leader who was in the audience, was delighted with Kennedy's answer. "Wow," he said, after Kennedy finished, "He endorsed our whole agenda." Wasserman pointed not only to the licensing answer, but also to Kennedy's statement in the prepared text that "it is time to end the licensing of new reactors."

Later, though, Kennedy's staff said the senator had not meant to rule out licenses for all plants now being built. The staff said Kennedy still supports a two-year moratorium, but is not willing to bar new licenses permanently.

Mondale rejected criticism from antinuclear forces in New Hampshire that the president had reneged on his 1976 campaign promise to make nuclear power "the last resort" in the energy mix.

Mondale said that was still Carter's view, and added: "It is our hope that through the remainder of this century increasingly, other sources of energy and energy conservation will bring us to the point we do not need nuclear energy."

In his speech today, Kennedy also rapped Carter for failing to stop exports of nuclear material and technology to foreign countries.

The senator's demcanor clouded noticeably today after a CBS-New York Times poll of Democrats showed that Kennedy has lost ground since the Georgetown University speech last month that was designed to resuscitate his campaign.

When he was first told that the poll shows him trailing Carter by 58 to 23 percent, he managed a grin and a quip: "Are you trying to make a grown man cry?" But later today there were no jokes and the major nuclear speech was given in a subdued manner.