President Carter will withdraw from the 1980 presidential campaign and throw his support to Vice President Mondale, a top Republican campaign strategist predicted yesterday.

John P. Sears, manager of Ronald Reagan's presidential drive, said Carter's political stock has sunk so low that it is impossible for him to make a comeback next year.

"He's at the point where everything he tries to do hurts him instead of helping him," Sears said at a breakfast meeting yesterday.

"By fall, Mr. Carter may have to stand up and say he's not going to run. There comes a time when such a decision is inevitable . . . and he is not far from it."

If Carter pulls out, Sears said, Mondale, a former Minnesota senator popular with the liberal wing of the Democratic Party would be "a unifying figure" who could avert a party-shattering battle for the nomination.

"Mr. Mondale has a large constituency of his own," Sears said. "He is not blamed for Mr. Carter's troubles. If Mr. Carter decides to step down, he would probably try to structure it to help Mr. Mondale."

Sears' remarks placed Reagan with the growing list of Republicans and Democrats who, for various reasons, are forecasting that Carter will not be his party's presidential candidate next year.

Sen. Bob Dole of Kansas, another Republican candidate, has been telling the GOP to ignore Carter and plan its campaign against Sen. Edward M. Kennedy (D-Mass.), who has said he is not a candidate.

Democratic Sens. Henry M. Jackson (Wash.) and George McGovern (S.D.) also have said they doubt the president can be renominated and reelected. They have predicted that Kennedy will be the party's 1980 nominee.