It's time to play "Do you mean to say?" -- the exciting new game show where contestants win fabulous prizes, like four years in the White House, just by misinterpreting one another. Get your opponent scrambling to explain himself and you could be the next president of the United States.

And if last night's TV debate -- Showdown on Main Street -- were the only presentation of issues and personalities in the current presidential campaign, then Jimmy Carter would probably walk away with all the moola, the Lincoln bedroom, the red telephone and those all-expense-paid vacations at Camp David. He won by making Ronald Reagan look flustered and bothered as he tried to defend his position.

In the course of this petty bickering about the future of the human race, the two candidates put on an entertaining if not fascinating television show. There was no 18-minute gap, no chronic lip sweating, and no acrobatic slips of the tongue about such matters as Soviet influence in Eastern Europe. But the differences in the candidates were underlined and reaffirmed in fairly graphic terms.

As TV personality, Carter looked cold, aloof and basically unflappable. Reagan let himself be backed into corners by Carter punches to the face and body and ended up repeatedly trying to defend statements he sometimes claimed never to have made in the first place. Reagan, usually as slick as the announcer on a soap opera, several times appeared uncomfortable and precarious, but he projected more warmth and spontaneity than Carter, who came across like the bionic peanut farmer.

Again and again Reagan let himself be tricked into covering tracks made by the president. On the matter of dealing with nuclear proliferation, Reagan said he had to "correct a misstatement of fact by the president." On the matter of nuclear energy, after Carter criticized the Reagan stand, Reagan said, "That of course if a misstatement of my position."

Reagan, after Carter criticism of his social security views: "Well, that just isn't true."

Reagan, to Carter, after Carter's critism of Reagan's Medicare views: "There you go again!"

Carter was also successful in repeatedly shifting the topic of discussion to nuclear war and the threat of doomsday, which the Carter campaign has painted as a more likely event should Reagan make his way into the White House. When ABC's Barbara Walters asked a question about a policy on terrorism, Carter tried to shift the focus to nuclear proliferation again and made a reference to "myself -- and the other leaders of the Western world." Later Walters chastised both candidates for failing to answer her question specifically.

It has been said that the only way the debate could have been decisive was if one candidate or the other made some colossal blunder that humiliated him in front of 100 million people and sent all the undecided voters scurrying to the opposite camp -- like the New Jersyites fleeing the imaginary Martian attack after the "War of the Worlds" broadcast. There was, however, no mistake quite so momentous. In fact, said NBC's John Chancellor after the debate, "There were no major mistakes that I could detect."

Chancellor also noted that by his own "smile count," Reagan had smiled four times and Carter only two. Heaven knows how many millions of minds were swayed by these crucial grins.

Perhaps fortunately, there were minor fluffs here and there. Reagan dramatically referred to "the muddy bloodled battlefields of Europe." Carter seemed to badly mangle a quote from H. L. Menchen. Reagan later said "missileman" when he meant "minuteman". Carter tried to make something of a Reagan reference to the days "before we knew we had a racial problem" in this country; but since the TV commentators didn't pick up on it later, that little gambit pretty well fizzled.

In a closing statement, Reagan made a reference to a "third candidate" -- John Anderson, not named -- who hadn't been allowed at the debate. But on Ted Turner's Cable News Network, Anderson, at Constitution Hall here, was being electronically interjected after each of the questions was answered by the two major candidates. Unfortunately, this didn't prove much beyond the fact that CNN is plagued with technical problems, as this was a very complicated maneuver to try to pull off.

More innovative and more functional, though also faintly scary, was the ABC News "Viewer Preference Poll" which allowed those watching to phone in their choices on who had won the debate so that the instant analysis could be done by the audience as well as the usual hired pundits. Apparently, however, the phone-in was also plagued with technical troubles, including jammed or overloaded lines, and there's reason to doubt the accuracy of it as a pulsetaking mechanism.

But this was the only new wrinkle in the debate format, kept rigidly formal and unnecessarily stuffy by the League of Women Voters, which sponsored the debate and chose to hold it in Cleveland.

Three other journalists, in addtion to Walters, interrogated the candidates, and under the unwieldy rules, several times had to repeat long, involved questions already asked once. Moderator Howard K. Smith cautioned the audience in the auditorium not to "applaud or express approval or disapproval" during the debate, and there were only two shots of the crowd, hidden in darkness, during the 90-minute broadcast.

If the audience is not allowed to make any noise or to be seen by the camera, what in the world are they doing there? Perhaps between now and 1984, the networks can devise a way to stage their own debates, and we won't have to wait until the last minute for a hastily arranged encounter like the one seen last night. It seems safe to assume that people will continue to rely chiefly on television for their information about political campaigns, and it is preposterous that the medium cannot be put to better use in disseminating information about matters to be decided in the voting booth. Television sets are as much a part of the democratic process now as are the voting booths themselves.

To the credit of both candidates they surmounted the obstacles imposed by the debate format, and after a genteel period of playing it cool, effectively mixed it up for the audience at home. It was no waltz, it was a real fight. Both candidates stayed on their feet, but Reagan was the one who most noticeably tottered. However, he was also the one who took the initiative on shaking hands after the match was over. This kind of sportsmanlike gesture may have had more impact on the viewers who saw it than anything that either man said during the debate itself.