A cynical crew of national political reporters was rendered speechless today by a nervous campaigner who injected actual human emotion into the plastic world of presidential politics.

Joan Kennedy, accompanying her husband to a press conference here, was asked -- it was the first question of the conference -- about recent stories questioning her husband's explanation of the Chappaquiddick tragedy.

She answered with a faltering but obviously heartfelt defense of her husband that was so moving nobody present could say a word when she finished. For a long minute, nobody asked another question. Another minute. Finally, everybody just walked away; the press conference was over.

"I wanted to ask [Edward M.] Kennedy about the Olympics," a network news correspondent said. "But you just couldn't do it after that."

It was not the content of Mrs. Kennedy's response that struck the press corps dumb -- she has said regularly that she believes her husband's explanation of the incident -- but her evident emotion as she struggled to articulate her dismay with the continuing reports challenging the senator's story. Even for hardened skeptics, it was impossible not to empathize with her.

Thus, nobody even questioned her suggestion that the timing of two of the latest Chappaquiddick articles was politically motivated. (When Edward Kennedy vaguely intimated the same thing earlier in the week, he was immediately challenged to provide proof.)

The rare spectacle of a one-question press conference occurred at the start of a series of airport appearances today designed to reach every media market in Iowa, where the first confrontation of the 1980 election campaign will be held Monday. Edward Kennedy stood at the microphone with his wife seated behind him.

But the first question was directed at her: in light of two new articles about Chappaquiddick this week (in Reader's Digest and The Washington Star), did Mrs. Kennedy believe her husband's story?

With apparent reluctance Joan Kennedy walked to the microphone. She paused for a while. "Yes," she said finally, "i believe my husband's story, which he told me right after the incident.

". . . and so I don't believe that these stories in the last few days are going to come up with anything new," she went on in a shakey voice. "I happen to believe that these stories are coming out now because of this crucial time, you know, the Iowa caucuses next Monday, and it's just too bad.

". . . we really should be discussing the important issues . . . it just seems a shame that, you know, it all has to come out again."

This was followed by utter silence.

As Joan walked back to her seat, her husband murmured, "Good. That was good," to her, but he did not immediately reach out to her. When it became evident that Mrs. Kennedy's response had ended the press conference, he put his arm around her and walked with her back to his campaign plane.

As Kennedy left the room, the silence was broken by a network producer. "Great television," she said.

At a later campaign stop in Cedar Rapids, Mrs. Kennedy, in a more spirited mood, appealed to Kennedy supporters to turn out for Monday's caucuses.

"That's where it's at, it seems to me," she said. "Teddy says, it's for the future of the country -- and it's for the future of Teddy."