Ronald Reagan today turned humbly to the man who defeated him four years ago and rode the broad political shoulders of Gerald R. Ford through a day of successful campaigning in Michigan.

In this western Michigan city that Ford represented in Congress for a quarter of a century, a crowd of more than 10,000 turned out to hear their favorite son ridicule Jimmy Carter's performance as president.

Reagan, for his part, said that Carter had failed because of "his total inability to fill Jerry Ford's shoes."

"President Ford gave us a world at peace, with America strong across the world and respected in foreign capitals," Reagan said. "He gave us an economy which had an inflation rate of only 4.8 percent. He left a sense of warmth and good feelings. He left unity and trust."

All Carter had to do, Reagan said, was to build on what his predecessor had accomplished. "And he wasn't intelligent enough to do it."

Ford's home-town partisans were reminded once again today that their hero is unchanged, that he remains the man his aides and friends call, with despairing affection, "good old Jerry Ford."

Introducing Reagan to the rally crowd on a chilly, sunswept morning here, Ford said the president has miscalled the city "Cedar Rapids" when he came here a week ago.

"Let me make this observation very respectfully," Ford said. "He's as screwed up in his geography as he has been in his economics." But a moment later, it was Ford whose geography was shaky. He said, "Grand Rapids was the most wonderful community in the 48 states," and then stopped and corrected himself to 50 states as the rally crowd laughed with and at the former president.

"I voted for Alaska and Hawaii," Ford added. "And I'm proud of it."

"He who lives by the sword, dies by the sword," a Reagan politician aide muttered in the background.

It was a mild gaffe for the man who once introduced Sen. S. I. Hiyakawa (R-Calif.) as Sen. Hiawatha." And it reminded one-time Ford political director Stu Spencer of the time in 1976 when Ford whistlestopped through seven Illinois towns and got the names wrong in four of them. Finally, aides gently suggested that Ford stop telling people the name of the town he thought he was in.

Later, in Battle Creek, Reagan got in the verbal misstatement act. Speaking about Carter's economic plan, Reagan said, "He promised tax increases," when he meant to say "decreases," and then explained: "I've been talking about Carter so long that I make mistakes like he does."

And Reagan told a group of demonstrators who were shouting "Bonzo" at him, "Well, they better watch out. Bonzo grew up to be King Kong."

Ford and Reagan did not begin this campaign as friends. But their mutual doubts are gone now. Time and time again, in private conversations as well as public statements, Reagan has praised Ford, who sat across from him on the candidate's campaign plane, and has expressed admiration for what Ford had accomplished.

Ford was effusive in his praise, too. When a reporter asked what had changed his opinion, Ford replied, "In the last six months, I've gotten to know Ron Reagan."

Former Ford political strategists Spencer and James Baker now occupy key positions in the Reagan campaign. Former Ford advisers like Alan Greenspan and George Shultz are expected to play significant roles in a Reagan administration. Even Henry A. Kissinger, never a favorite in Reagan circles, is now discussed in tones of courtesy and respect.

Ford's good humor was matched on the Reagan plane, where the mood is now one of anticipated victory next Tuesday.

Aides who only a few days ago were reluctant to think beyond Tuesday are now making plans to move to Washington. The plane takes off to the recorded messages of Carter extolling the virtues of his daughter, Amy, or of Bible-reading in Spanish with Rosalynn. Cameramen sing the Roy Rogers song, "Happy Trails," to Reagan as he goes to his hotel room at night. Reagan, in a drop-in visit to supporters, discarded his usual caution and said, "We're going to win this thing."

This mood shows in the growing Reagan crowds, too, where supporters now press in on the candidate in what sometimes seems an almost atavistic urge to touch their leader.

The confidence was boosted by new polls taken by Richard Wirthlin, who today said the undecided vote was breaking 3 to 2 for Reagan. The only cloud in sight in the silver lining of an anticipated Reagan victory remains the unresolved hostage issue.