Sen. Edmund S. Muskie (D-Maine) said yesterday he senses a growing feeling among top Democratic leaders that President Carter's political stock is dropping so rapidly now "that he may not be able to recover."

Muskie made his remarks at a breakfast meeting with reporters at which he declared Sen. Edward M. Kennedy (D-mass.) as "probably" the stronger candidate, and said Kennedy has shown in recent weeks "he realizes there is a center."

However, Muskie stopped well short of backing Kennedy for the Democratic presidential nomination, contending it still is "a little too early" to decide and saying he still is determined to "help the president succeed."

Muskie said "the best course for the Democrats is for the president to maintain his political viability as long as he can. If that seems at odds with Sen. Kennedy's view of things, I'm sorry."

For all Muskie's professions of loyalty to Carter, his remarks yesterday constituted a devastating assessment of the president's prospects by a major mainstream Democrat.

The 1972 presidential hopeful denied that a challenge by Kennedy would prove divisive to the party. "It's going to divide people, but it need not be divisive," he said. "It probably undercuts the president's political base."

Muskie's comments came in response to a question on whether he now sees a "stampede" among Democrats to back Kennedy. Muskie said it had not reached such proportions, but conceded Carter's stock was "dropping" while Kennedy's was up.

It was then that the Maine Democrat said "there's a feeling about Carter that he's losing his public support at such a rate that he may not be able to recover." Asked if Kennedy is the stronger candidate, he said: "Probably."

Muskie's comment that Kennedy may be moving toward the center politically was based on the Massachusetts senator's recent votes on the budget resolution. Kennedy voted for a $3.6 million cut in domestic spending to hold the deficit down.

Muskie, who as chairman of the Senate Budget Committee was leading the cutback drive, said Kennedy's votes "suggest to me he realizes there is a center." He implied, without saying so, that Kennedy in earlier years might have tried to challenge the cutbacks. He said the budget measure would have provided an opportunity "for anyone who wanted to get visibility."

Speaking of Carter, Muskie said he and most other members of Congress have been "impressed" with "the thoroughness of his homework" and the way the president tackles a job, but conceded Carter often was "confusing."

However, he stopped short of criticizing the president directly on specific issues.Muskie praised Carter for dampening pressures for a 1980 tax cut, saying he agreed that Congress should wait until January before deciding.