Is Ted Kennedy fixing to challenge Jimmy Carter for the Democratic presidential nomination in 1980? A lot of signs point that way.

But a couple of recent chats with the senator from Massachusetts persuade me that he has no such design. While circumstances could conceivably force him into the race, he presently feels overexposed and is confining his campaigning this year to a bare minimum.

Of course, those always suspicious of the Kennedy clan and those keen to finish off Jimmy Carter can find much contrary evidence. Kennedy broke with the administration on national health insurance and the natural-gas bill as Carter hit a new low in the public-opinion polls.

A couple of favorable magazine articles on Joan Kennedy gave credence to the theory that the sentor was trying to preempt in advance the "moral" issue that has shattered him since Chappaquiddick. On his recent trip to Russia, he showed he could get out of the Soviets more than the president or his emissaries.

But taken one by one, those incidents add up to much less than a game plan. Mrs. Kennedy initiated the articles to explain why she could not be campaigning with her husband this fall.

Kennedy himself has made national health insurance a personal campaign, and felt he would lose credit with his liberal and labor backers if he acceded quietly to its burial. Connections with consumer groups determined his stand on the natural-gas bill.

But on that issue - where, as the Republicans realize, Carter could be mortally wounded - Kennedy has not lobbied against the administration. "We don't find Kenndy footprints anywhere," John McMillian, a pipeline operator who is probably the most effective lobbyist for the bill, said the other day. "We think we'll have the support of most of the New England senators."

The Russians did give Kennedy assurances respecting the release of certain dissidents that they had refused the Carter administration. President Leonid Brezhnev received Kennedy even as he was refusing to see Carter's disarmament negotiator, Paul Warnke. But that was not the doing of Kennedy, who planned his trip over a year ago in connection with a health conference in Alma Ata.

To be sure, there is Carter's low standing in the polls, even when running head-to-head with Kennedy. Kennedy no longer believes, as he used to, that Carter is a shoo-in for renomination and reelections.

If some third Democrat looked that he might toppled Carter, moreover, Kennedy would not comfortably sit on his hands. But it isn't as though just any Democrat could do that easily. To do what Gene McCarthy did to Lyndon Johnson in 1968 would require a pure protest candidate, one who is not taken seriously himself but merely gives the voters a chance to express anti-Carter feeling.

But candidates of that stripe are extremely rare - especially when there is no overwhelming protest issue, such as the Vietnam War. Both Jerry Brown, the California governor, and Daniel Moynihan, the New York senator, are too well know as persons and on the issues to be merely protest candidates. And after them, who is there?

Certainly there is no evidence Kennedy is positioning hinself on the theory that some third candidate will force him to jump into the race. On the contrary, the stances he has been taking on such matters as health insurance and detente are far too liberal for a national constituency that is edging to the conservative side.

Kennedy's campaigning schedule this fall, far from being calculated to build up political IOUs, is remarkably limited. Apart from Massachusetts, he is appearing only in behalf of two candidates for governor - Peter Flaherty in Pennsylvania and Richard Celeste in Ohio - and seven candidates for the Senate.

Most important of all, there still remains the major deterrent: the danger of another tragic disaster to the head of the Kennedy family. At the very least it seems doubtful to me that Edward Kennedy would actively seek the presidency while his mother, Rose Kennedy, is still alive.

The more so as he seems as happy with the role he now enjoys as a liberal leader in the Senate, the coming head of its Judiciary Committee and a man sometimes able to be free from care. One of the first calls he made the other day when he returned to Washington from Russia was to his old friend Speaker Thomas P. (Tip) O'Neill. The only thing close to a game plan they discussed was how come the Red Sox had lost four in a row to the Yankees.