The Democratic Party began facing up to a "Kennedy Gap" yesterday, with most officials guessing it would enable former vice president Walter F. Mondale and Sen. John Glenn to move up in the competition among the party's presidential candidates but allow more room to maneuver for longshot and late-starting candidates.

The withdrawal of Sen. Edward M. Kennedy from the 1984 contest set off an instant scramble among the other contenders to sign up his supporters.

Sen. Alan Cranston of California, portraying himself as the closest to a Kennedy liberal left in the race, said he started early yesterday morning telephoning those who had told him they would be glad to help him if Kennedy were not running. "A number said 'We're ready,' " Cranston said, while others said they needed time to recover from their "shock."

A Mondale aide said the Minnesotan and his assistants also were working the phones to Kennedy supporters.

The gap that Kennedy left on the liberal wing of the party may soon draw new contestants into the race.

Rep. Morris K. Udall of Arizona, Jimmy Carter's most persistent liberal challenger in the 1976 primaries and a 1980 Kennedy supporter, said in Los Angeles that Kennedy's decision "has forced me to reconsider" staying on the sidelines.

"My operating theory was that there was no scenario that could force me in, back when Kennedy was in the race," Udall said during a visit to the National League of Cities Convention in Los Angeles. "But this has forced me to reconsider."

Sen. Dale Bumpers of Arkansas, a partner of Kennedy's on much legislation aimed at closing tax "loopholes," said he was "keeping his options open" because "there's a pretty big vacuum out there."

There also was speculation about half a dozen other senators and former senator George S. McGovern, the defeated 1972 nominee, but none made any overt move to jump into the race.

President Reagan, visiting Brazil, said he was surprised at Kennedy's announcement but disclaimed any intention of following suit. "You know," he quipped, "I don't believe there is much of a record of me imitating Teddy Kennedy."

Carter, who turned back Kennedy's challenge for the 1980 nomination and made it clear that he opposed him as the 1984 nominee, said Kennedy's withdrawal "doesn't change my plans. I'm not a candidate and I have no plans to run for office again."

Carter, interviewed in Atlanta, said he was sure "the other candidates will be relieved that this formidable candidate will be removed."

That feeling was best expressed by Glenn, who said "any time someone ahead of you in the polls drops out, that's a plus."

That sentiment crept into the tributes to Kennedy by Mondale, Glenn, Cranston and the other all-but declared challengers, Sens. Gary Hart of Colorado and Ernest F. Hollings of South Carolina and former Florida governor Reubin Askew.

Most agreed with Hollings' statement in Los Angeles that "obviously, Mondale is the big gainer. The question now is whether he can retain the gain."

Mondale is closest to Kennedy in the preference polls of Democratic voters and is regarded by his rivals as being the most advanced in preparations to field and finance a full-scale drive for the nomination.

With Kennedy's withdrawal, Mondale becomes the almost automatic favorite for the two major organizational endorsements that may precede the first primary -- those of the National Education Association and the AFL-CIO.

Glenn Watts, president of the Communications Workers of America, said that Mondale "is far and away the front-runner at the moment" for the AFL-CIO endorsement. Douglas A. Fraser, retiring president of the United Auto Workers, said Kennedy's withdrawal eases the conflicts for unions like his that "were dreading the primaries where they'd have to choose up sides" between Kennedy and Mondale.

But with the favorite's role for Mondale, many Democrats were quick to point out, would go the attendant risk of a stumble from which such past front-runners as Edmund S. Muskie have been unable to recover.

AFL-CIO spokesman Murray Seeger noted that Kennedy's action "puts a tremendous load on Mondale's shoulders" while opening "the field to some people who didn't think they had a ghost of a chance a week ago."

Mondale, according to several Democrats who spoke with him during the day, seemed ebullient about his chances and more than ready to run the risks of being annointed the favorite.

A close aide observed that "Mondale has had the burden of being treated almost as front-runner for many months, without having any of the advantages. We get all the scrutiny from the press and other politicians," he said, but because of Kennedy's lead in the polls, Mondale had no leverage to pressure anyone to "get on board" his bandwagon. Now, he implied, that may change.

But in Iowa, site of the first caucus, party chairman Dave Nagle said Mondale "has never had a lock on this state and Kennedy's withdrawal opens it up even more." And in New Hampshire, where the first primary will be held, state Sen. Richard Boyer, the Democratic chairman, said Kennedy's withdrawal "helps all the other candidates more than Mondale," particularly those, like Cranston, who can claim to represent the same liberal causes.

A prominent pro-Kennedy Philadelphia Democrat, mentioned as a likely early convert to Mondale, said he was going to be very slow about making up his mind. Several Democratic officials speculated that Mondale might lose support among those who favored him as the likeliest stop-Kennedy candidate, but would reassess their position now that the "threat" of a Kennedy candidacy was ended.

Meanwhile, Glenn made his own move to pick up the Kennedy vote, recalling in interviews during the day that he had campaigned for the nomination of Robert F. Kennedy and was with him when he was murdered in Los Angeles in 1968.

Robert J. Keefe, a Glenn adviser, said that with Kennedy out of the picture, Glenn could use his Ohio base -- with its strong union membership -- to mount a serious challenge to Mondale's endorsement by the AFL-CIO.

But Keefe conceded a point made by many other Democrats that for the Glenn camp, still in the early stages of national organization and fund-raising, Kennedy's withdrawal "presents opportunities we are ill-prepared to exploit."