An unsolicited suggestion for Walter Mondale: Half an hour before your debate with President Reagan is to take place, call in sick. Let Jesse Jackson take your place.

Please don't take this as an insult. I'm not saying that you don't understand the issues. You understand them in greater detail than does Jackson, and, having been around national politics longer, you have a better understanding of what is possible for an administration to accomplish.

My point is that such knowledge isn't particularly helpful in a debate with Reagan. Reagan's strength, as a hapless Jimmy Carter found out, is not how much he knows about the nuts and bolts of government but how well he understands people: understands that they would rather hear inspirational myths about themselves and their country than entertain depressing truths.

They don't really want to hear how wonderful they and their country could be "if only . . ." because they know from experience that the "if only" isn't going to happen. They want to be told how wonderful they are right now.

All that will strike you as hypocrisy (which is why you should develop laryngitis before the debates begin) but it would be just another day's work for Jesse Jackson.

Think about it. What is the recurring theme of Reagan's speeches? That America is great and good and strong, and that its people are in good shape. What is Jackson's motif? "I AM somebody." And what is yours? That the country is in lousy shape, its values confused, its people hurting, its future mortgaged. No matter that you are closer to the truth; the voters don't like crepehangers.

Your focus is short, reaching back only three or four years, and its emphasis is on problems. Reagan and Jackson take the longer view, and their emphasis is on strength. They see ponies. You see only that the stable needs cleaning. (It does, and you are far more likely than Reagan to start shoveling. But it's not a pleasant thing to talk about.)

Reagan defines his constituency in terms of the good old days of barn-raising cooperativeness rampant entrepreneurship. They ARE somebody, he tells them, never mind that the barn- raisers killed off the Indians and ravaged the land, or that the entrepreneurs included robber barons, horse thieves and lynch mobs. Jackson defines his constituency in terms of obstacles valiantly overcome and progress nobly made, never mind that some of them were more hindrances than engineers of that progress. Both men make their constituents feel good about themselves, their history and their prospects.

They lead their congregations in stirring songs that lift the spirits. You give them sad-eyed announcements of leaking roofs, inadequate collections and impending foreclosure. There may be truth in what you say, but there's no music in it.

What Reagan and Jackson do best of all is to give their people a sweeping vision of their world and their place in it. The specifics aren't always accurate, but the broad vision rings true. Your facts are solid, but one hardly knows what your broad vision is. Jackson and Reagan inspire their people with feel-good one-liners; you put yours to sleep with staff-written white papers.

You think the American people are bright enough to see through the happy talk and deal with substance. You're wrong. To take just one example, Reagan utters pieties about the American family; you have a strong family. But who gets credit for his efforts to restore the sanctity of family? Reagan, who might be hard- pressed to tell you what street his children live on or the name of his newest grandchild.

You're a good man, and you have the potential of being a better-than-average president. But I wouldn't bet a dime on you in the debates. My advice is: Call in sick and let Jesse do it.