After Sunday's semi-debate in Baltimore, Ronald Reagan seemed to emerge as the biggest winner -- largely because he wasn't trying to win at all. e

Reagan suceeded in his major goal: he cast himself in the role of a calm and decent ex-governor, aimed his appeal at that sizable segment of voters who might support him but who have expressed doubts in polls concerning his stability and capability; and he played his part well.

John Anderson had a politically profitable night, too, seeming to solidify what had been an eroding standing among liberals with some strong, and at times strident, attacks on Reagan from the left. But Anderson's gains, whatever they might be, are likely to come at expense of the evening's candidate in absentia, President Carter.

And because of this, Reagan emerges as the major beneficiary of Anderson's success. Which is just the way the Reagan men had if figured all along.

In the wee hours after midnight Sunday, long after their candidate had gone to bed, the members of the Reagan high command sat in a suite on the 16th floor of the Baltimore Hilton, their jackets cast aside, their ties loosened, and talked in words of sweet satisfaction -- and undisguised relief -- about the way things turned out in that controlled panel interview show that the League of Women Voters billed as a debate.

"The governor accomplished all of his objectives," said Richard Wirthlin, Regan's pollster and chief strategist. "A political debate is a high-risk thing. But it turned out just fine. We weren't trying to win a debate against John Anderson. So Anderson came off fairly well . . . which was fine with us."

In another corner, William Casey, campaign manager and elder eminence of the Reagan inner circle (he is, at 67, just two years younger than his candidate), alluded to what had been the Reagan camp's major unspoken concern as their candidate entered the debate.

"We didn't hurt ourselves," he said.

He went on: "Ronald Reagan was able to show a certain warmth. He handled some tough situations and there was no problem. He showed that he is not a hard, flinty conservative -- that he doesn't have a finger that's just itching for that button."

For President Carter, the Sunday night offering by the League of Women Voters was always an exercise at cutting his losses. The event was never considered in terms of being a net win, by the Carter strategists. As they saw it, independent Anderson would reap large gains just by being treated as a full-fledged equal in a three-way debate -- and those gains would come at Carter's expense, sizable enough, they feared, to perhaps cause Carter to lose his margin of victory over Reagan in some Northeast states.

Thus Carter could do very well in a three-way debate and still wind up a net loser on election day, the president's advisers concluded.

So the president opted to gamble. He chose to pass up the debate, proclaim it just a debate among Republicans, and hope that Reagan and Anderson cut each other up enough to minimize the damage caused by his absence.

But as the debate played out, Anderson proved to be the perfect foil for Reagan, and Reagan was the same for Anderson. Each enabled the other to shore up his own sagging constituency, with minimal losses enroute. All of which meant that Sunday was not a good night for Carter -- even though the two Republicans on stage focused relatively minimal artillery upon attacks against the incumbent's record.

One brief exchange during the televised debate illustrated well how the Reagan and Anderson battle plans complemented each other even as they enacted their attacks.

Anderson going after the conservative Republican nominee just the way his liberal supporters hoped he would, charged that Reagan's plan to cut taxes and still balance the budget ignores his promises for nuge military expenditures. Anderson, his eyes flashing intensely behind his magnifying dark-rimmed glasses, ran through the litany of Reagan plans for a new manned bomber, a permanent fleet in the Indian Ocean, for a modern aircraft interceptor.

". . . I have seen his program costed out to the point where it would amount to more than $300 million a year just for the military," Anderson charged, in his trademark gatlinggun delivery. "And I think the figures he has given are simply not going to stand up."

Reagan kind of paused, shook his head, and in a aw-shucks way, took quiet issue with the man he would call -- always respectfully -- just plain "John."

"Yes," Reagan shucked, "some people look up figures and some people make up figures. And John has just made up some very interesting figures." He went on to talk about how John doesn't realize . . . " and if "John were not listening closely, he might have thought, from the dispassionate and downright friendly tone of it, that he was being praised, not put down. This was the most rancorous of their exchanges. Throughout the night, the two men mostly repeated their campaign stump spiels. Anderson made points with his constituency by trying to challenge Reagan's facts in a hard-charging, almost baiting way; but Reagan would not rise to debate.

This was according to the Reagan plan. As his chief of staff, Edwin Meese, observed: "The contrast with Anderson was helpful. Anderson was more strident, more rigid. It enabled the governor to come on more low key, and he showed a concern for people -- which is something we wanted very much to do."

As pollster Wirthlin explained, in the language of his profession that has come, in this age of automation, to be the language of journalists as well: "Ronald Reagan was trying to speak to a large number of people who are now in an active information-seeking mode." These are people who are telling Withlin's field poll-takers that they are not strongly opposed to Reagan, but they want to know more about him.

"We gave a large number of people the opportunity to see that the governor doesn't have horns," Wirthlin said. "If the debate was rancorous, it could have defeated out other objectives. And if we beat up on Carter too much, in his absence, there was the danger of creating a sympathetic backlash for him."

"So it turned out well for us and for Anderson, too. After all, very few people are trying to decide between Reagan and Anderson."

Far more people are trying to decide between Carter and Anderson, the Reagan men know. And that is why it is difficult for the experts in all camps to say whether Carter's gamble in taking a pass on the debate will turn out to be a political disaster or a loss-cutting success.