Campaigning for her husband in Brooklyn last Thursday, Nancy Reagan confided to a friend that, yes indeed, Ronald Reagan should debate President Carter head-to-head, a confrontation, she added, that Reagan would win hands down.

With such patronage, debate-prone activists in the Reagan campaign plowed ahead with a three-pronged strategy to exorcise Reaganemia and sharpen a campaign that, as one told us, has "plateaued out at a level too low for comfort." Reagan, in short, has given voters sufficient reasons for not supporting Carter but he has not yet convinced them to take the plunge and vote for him. The decision to debate was the essential first step. It followed a comparatively fruitful day of campaigning in crucial Michigan. That netted old-time civil rights leader Ralph Abernathy and two other black leaders as a hedge against the Carter administration's "you're a racist" attack, but fell short on inspirational content needed to convert large numbers of Carter-shunning Democrats from indecision to active support of Reagan.

Debate-pushers in the Reagan camp found an unexpected asset in the glittering Al Smith dinner in the Waldorf Thursday night. Ignoring the unwritten rule against serious politics and speaking after Reagan's gentle self-ridicule, Carter typically tried to take advantage of his opponent with such gracelessness that the overwhelmingly Democratic audience gave him a Bronx cheer. "If that's the real Carter," one Manhattan Democrat said after the dinner, "why doesn't Reagan take him on in a TV debate? Carter makes him look positively genial."

Reagan operatives believe, not without reason, that the sharp contrast in personality -- "genial" Ron versus uptight Jimmy -- gives Reagan a head start in their momentous confrontation.

The second prong of Reagan's ninth inning effort to shake undecided voters into his basket are up to three half-hour nationally televised talks, the first of which was last night. The second is planned for late October, the last (to be broadcast over two networks if money and schedules permit) on election eve.

The third prong of Reagan's last-ditch offensive is a series of five-minute paid TV commercials done on the road with Carter's economic miseries the main target. Reagan's traveling aides were bubbling at the first of these, done in Lima, Ohio, last week, not by Peter Dailey, Reagan's regular producer of TV ads but by independent Los Angeles producer Bill Carruthers under the direction of Jim Baker, the Reagan camp's debate maestro.

The Lima road-show commercial, while still hitting at Carter with only medium-boiled hardness, was clearly superior to the fluff of the past months. Those to come will increase in intensity.

The case for both the debate and a sharper Reagan cutting edge in the last two weeks of a presidential campaign that threatens to set new boredom records is strengthened by Carter's maneuvers on the hostage front. Taxiing to LaGuardia airport Friday morning to catch the Washington shuttle, top Reagan aides William Casey and Edwin Meese discussed what, if any, advantage Carter would gain from pre-election hostage release. Reaching for pie in the sky, they figured that voter cynicism born of Carter's crude use of the hostage issue during the primaries might limit the political impact of what otherwise should be a tour de force.

But on second thought, Meese and Casey undermine their own case by agreeing that release of the hostages would be a "wild card" -- the highest card in the deck.

With Reagan's strength in the critical industrial states at a level well under 50 percent and the true extent of his southern pick-up still questionable, Carter's "wild card" could be played with devastating impact just before the polls open. "

That is one reason underlying the debate decision: a win over Carter would give Reagan a cushion to absorb the impact of a possible break for the president on the hostages. But the more important reason is Reagan's plateau. It is clearly too low for comfort at a time when the challenger should be riding high.