TO JIMMY CARTER:

What a magnificent comeback you have so brilliantly engineered. Poor Ronald Reagan -- he must be unhinged by the thunder of hoofbeats (yours) closing in on him. Here are my recommendations for the remaining days. 1) Unleash Fritz Mondale.

You must muster the most persuasive case for your reelection. Your vice president is, for many of the voters you must win, both your most effective messenger and your most compelling message.

In a campaign bereft of humor, Mondale is comfortable with that commodity and, most important, Mondale's humor makes other people comfortable.

In the closing days of all presidential campaigns, perspective is a stranger on the candidate's plane; judgment is generally absent from strategy sessions. As proved by his gut reaction to Reagan's switch on the hostage issue, Mondale remains on a first-name basis with both judgment and perspective.

Adroitly challenging Reagan to reveal his "idea that he thinks can help" win the release of the 52 Americans, your vice president wisely did not personalize the issue; he simply turned the focus back on the Republican.

Surely, if the Reagan campaign can buy time to put George Bush on television, the Democrats can squeeze out a few dollars to enable Fritz to make his commercial TV debut for the Carter-Mondale ticket. Please call Bob Strauss on this. 2) Think Big D.

Mention of Bob Strauss and Fritz Mondale leads directly to the key to your victory on Nov. 4: the Democrats. In George Gallup's most recent colloquy with the American voters, he learned that 48 percent of them consider themselves to be Democrats, while only 25 percent admit to being Republicans. Yet, according to Gallup, Reagan leads you among all voters by 45 percent to 42 percent. Winning means winning back the Democrats.

Keep chanting that Democratic mantra -- Roosevelt, Truman, JFK, Humphrey and (where appropriate) LBJ.

Joint appearances with Ted Kennedy have been helpful. But greater restraint on your part is urged. After your springtime unpleasantness with him, it's hard for some voters to believe that you are dying to double-date with him. 3) Give Peace a Chance.

The peace issue works a lot better for you in 1980 than it did for George McGovern in 1972. Stay with it; don't go near inflation or interest rates. Make the undecideds decide on war and peace. For you. TO RONALD REAGAN:

What can I say? You're beautiful. You're ahead. You will win. Ciao.

1) Ignore Anybody With a Quick Fix.

Trust your own instincts completely; they brought you where you are today. For example, one quasi-genius wanted the Reagan-Bush campaign to sponsor, in the top 40 media markets, on the Sunday before Election Day, the televised Mass for the Shut-Ins. No. No more Theology 101 and whose prayers get answered first and who has to take a number.

Be yourself. You are likable, but in the debate be a little more Nashua and a little less Baltimore. 2) Work the Economy.

Every day that the economy is not the day's lead political story is a victory for Jimmy Carter. Americans want to elect a strong, competent leader -- not a revisionist historian. Carter's essential message, which has worked, has been: "I may be no day at the beach myself, but the other fellow" -- you -- "could be dangerous to your health."

The race is still up for grabs. You must be on the offensive about the economy, taxes, inflation, interest rates, jobs. 3) Here is The Winning Idea.

Buy half an hour of network time, next Sunday night, for former president Gerald Ford. He can provide you with Innocence by Association on the war-and-peace question and on the Middle East.

In addition, he, better than anyone else, can do for you what Ed Muskie did for the Democrats on election eve 1970 when he rebutted so eloquently the charges of Nixon and Agnew. He may even want to think about quoting from that speech: "They really believe that if they can make you afraid enough . . . you can be tricked into voting against yourself." Think about it, anyway. TO BOTH CANDIDATES:

Pennsylvania is a key state. The Phillies did something good this week. So please remember: "Tug" McGraw is not a misprint and Dallas Green is not a condominium with prices starting in the low 70s.