President Carter has "reached rock bottom" in the popularity polls and will soon show signs of rallying the Democratic Party around his reelection in 1980, Rep. Morris Udall (D-Ariz.) predicted yesterday.

But if Sen. Edward M. Kennedy (D-Mass.) decides to run, "all bets are off," Udall said.

Udall, one of the leaders of the Democrats' liberal wing, declined to choose between the two men, but told a breakfast meeting of reporters that he expects to be backing Carter by the time of the election.

The party is beginning to coalesce around [Carter]," Udall said. "I just sense it. There's no real proof . . . it's the talk in the cloakroom."

Opposition to Carter's renomination lacks the bitterness and outrage, "the willingness to hand the White House to the opposition over one issue," that forced Lyndon Johnson to bow out in 1968, Udall continued.

Carter would have to fall badly in the polls behind Republican leaders, including former California Govenors Ronald Reagan and former Texas governor John Connally in order to be branded an irretrievable loser, " and I don't see that happening," Udall said. "I think he's reached rock botton already."

Only the worsening economic situation could spell real trouble for the president since he has not built himself a loyal following that is with him whatever happens, Udall said. Carter asked for Udall's advice in a recent meeting, the congressman confirmed, but did not ask for a public endorsement nor for an agenda to recapture the liberal vote.

"He's a quick learner . . . a very determined man . . . the best one-on-one campaigner this country has seen in a long, long time," Udall declared. "I'd be shocked and extremely surprised if he didn't" seek reelection.

While unwilling to endorse Carter over Kennedy, Udall did not hesitate to back the president over California Gov. Edmund G. (Jerry) Brown Jr. "If he's the candidate against Jimmy Carter, it makes it easy for me," Udall said.

Brown, he added, could make "A very strong showing" in the New Hampshire primary race by forging a coalition of anti-nuclear power activists on the left and advocates of a nationwide constitutional convention on the right, both strong currents in that New England state. "You get 30,000 to 40,000 votes in New Hampshire and you're a major figure," Udall said. both heard Carter make the remark and repeat it when asked to do so.

The White House dinner was Monday night for some 60 members of Congress, who were also given a briefing on implementing legislation for the Panama Canal treaties.

Downey, who said he was sitting immediately on Carter's left, said the president was asked how he felt about the 1980 presidential race. Downey quoted Carter as saying, "I feel good. I'm ready."

Downey said someone at the table then asked about Kennedy, to which Carter was quoted as replying, "If Kennedy runs, I'll whip his ass."

Brodhead said, "I was startled. I thought my ears were deceiving me. [I said] Excuse me Mr. President, what did you say? And Carter replied, 'If Kennedy runs, I'll whip his ass.'"

Both Brodhead and Downey were reached by telephone last night.

Asked about the statement, White House spokesman Mark Henderson replied, "I have no information on that," and said the White House would have no further comment until today.

Neither the president nor Kennedy is a declared candidate for the 1980 Democratic nomination. Even though Kennedy has said repeatedly he thinks Carter will get the nomination and that he would support him, various "draft Kennedy" movements have popped up around the country.

Carter has said in recent months he can defeat any challenger for the nomination.