When Johnny Carson makes fun of a public figure, it is generally safe to assume that the person has become fair game. The theory is that when Carson is ready to kid, Middle America is ready, too.

But several nights after the Mondale-Reagan debate, Carson attempted a few pleasantries about President Reagan's undisputed flop in Louisville. He noted that when the next debate takes place, Reagan "will be two weeks older."

Instead of the cascade of laughter that he usually gets from his doting audience, Carson was met with heavy silence.

He pressed on. "Mondale is on a roll," he said, to more silence. Then he added, "So were the Cubs." That went down a little better.

If the "Tonight" audience is the electorate writ small, American voters are terribly sensitive about Reagan on the ropes. They were embarrassed and disturbed by seeing 100 unedited minutes of the man they intend to reelect.

They could not handle the sight of their glossy, easy-talking president completely taken aback, staring at his challenger as if he were an intruder who had burst into the White House living quarters.

Mondale was a genial, informed, gracious person they had hardly seen before. He was a suitor the electorate had turned down with scarcely a moment's thought. They didn't like the people he hung around with -- Jimmy Carter, Bert Lance, Lane Kirkland, the poor, the minorities. He talked about taxes and other unpleasant things. They didn't like his voice, the way he looked on television. He was a loser, so they went on to other things.

The electorate is like a girl with an engagement ring. When a rejected suitor comes forward and shows up her fiance, she doesn't snatch off her diamond and throw herself into the arms of the messenger with the bad news.

She reacts defensively, protectively. So he had an off night -- so what? She knows he doesn't always get the facts straight -- but who does? Maybe he squanders money on weapons -- but look what he did for the economy. Yes, he should talk to the Soviets more than once every four years -- but who could think that such a nice man would start a nuclear war?

Some dangling Democrats -- not enough to make a difference, according to the president's managers -- made up their minds on the spot. Leaving their party for a masterly, in-command world leader was one thing. But leaving it for a man who couldn't even say why he should be reelected was quite another.

But the usual thing is happening in Reagan's case. Just as they forgave him for making a gag about bombing the Soviet Union, for comparing the incomplete security precautions in Beirut to a delayed kitchen remodeling, for rehiring Anne Burford, for weakening public education and fooling around with Social Security, they were receptive to hearing excuses about his bad performance.

And he is going to have another chance. At a second debate he will come back. He will be rested, free of the pesky facts that clogged his Louisville delivery. He will mouth the slogans they love so well about "respect for America." He will wave the flag, rattle the saber, yearn for peace and make everyone feel good all over again.

Robert Strauss once observed that Reagan walks away from a car crash that no other politician could possibly survive. On Oct. 7, Reagan totaled his campaign. Alibis flowed from his camp. He was "brutalized" by the briefers. He had been overscheduled. He really had nothing to do with it.

His managers say that while Mondale may have gained in the esteem of voters, he gained few votes. If Mondale's favorability ratings shot up, Reagan's held steady. Of course, if Reagan bombs again in Kansas City, that could all change.

A reporter traveling with Vice President Bush last week picked up random comments that show how difficult it is for voters to ditch "Dutch."

A Philadelphia taxi driver, wearing a 10-gallon hat and a chip on his shoulder, was incensed at post-debate talk that Reagan might be too old for the job.

"They're trying to put him down because he's 73," he growled. "That's discrimination. He had a bad night -- who doesn't?"

An Italian-born dockworker in New Jersey said it didn't bother him to see Reagan looking old. "In my country we have a saying, 'An old chicken makes good broth.' I still like him because there are too many make-believe poor on welfare."

A Republican woman in Alabama, wholly committed to the Reagan-Bush ticket, said she was "upset" by Reagan's first debate. "He was awful. But he has a lot on his mind."