"Ted Kennedy made a mistake. He didn't have to become a candidate so soon. So Jimmy Carter now has a new opportunity. But I hever cease to wonder at his inability to see where he is politicaly."

John Sear, Ronald Regan's campaign manager, made those comments in a chat here the other day. His opinion is surely not disinterested, and not surely right. But it bears exposition in detail as a view from the other side, undazzled by the glamour of the occasion, which provides a measure for judging the big event now shaping up between Kennedy and Carter.

Kennedy, according to Sears, has all along been the front-runner for the Democratic nomination. Sears rightly claims to know a lot about front-runners. He has stewarded Regan's effort to stay at the top this time. He also figured importantly in Regan's nearly successful effort to overtake Gerald Ford for the Republican nomination in 1976.

"The business of a front-runner," Sears said, "is to keep moving forward at his own pace. As long as Kennedy was not a candidate, he was advancing nicely. If Carter complained about the senator, it sounded like sour grapes. So the president had to keep talkng about the economy and energy and the Congress and foreign policy. He was on the defensive.

"By declaring now," Sears continued, "Kennedy gives Carter a legitimate reason to attack him Kennedy becomes the issue, not Carter. That's why Carter now has a new shot."

I asked Sears if Kennedy hadn't been obliged to declare by pressures from partisans who might otherwise have gone elsewhere. Sears said:

"Kennedy not only leads Carter in the polls. He has one thing Carter lacks -- a base. The Kennedy constituency is really for Kennedy. They'll vote for him over much better men than Jimmy Carter, and they'll vote for him whenever he announces. He could have waited until January. He could have won New Hampshire on a write-in."

I asked Sears what Carter had to do to make the most of his opportunity. Sears said:

"It's not enough to make allusions to Chappaquiddick, or to hint that Kennedy will split the party. Carter has to take actions that force Kennedy to position himself on the left.

"For example, he might go to Boston, or Manchester, N.H., the heart of Kennedy country. There he takes a position that is unpopular locally.

"Let's say he comes out for total and immediate deregulation of gasoline prices. That kills him in Massachusetts and New Hampshire. But he's dead there anyway.

"The beauty of the thing is that Kennedy has to come out strong against that, of course. When the primaries move to the South, Kennedy is in trouble. He's stuck hard on control, and Carter has an issue for Texas, Louisiana Alabama, Mississippi, Florida and South Carolina.

"All Carter needs to do is hang on for a while. Then he can go to the West on another issue -- say, water -- where he can show that Kennedy is more to the left than he is. Then he can do the same thing in other sections and so forth and so on."

Two considerations follow from that analysis. The first is that Kennedy needs a quick victory. Otherwise he risks a long, drawn-out war of attrition that will cut him down to mortal size and split and demoralize the Democratic Party. Fortunately for Kennedy, the elements for a quick win are at hand.

He can start strong in the Iowa caucuses in January and grow stronger in the New Hampshire and Massachusetts primaries at the end of February and in the first week of March. A win in South Carolina or Florida or Alabama or Mississippi in the second week of March would show Carter's vulnerability in his home base. Then in the next month Kennedy can tie it down by winning in Illinois, Pennsylvania and New York.

The second consideration is that Kennedy has to avoid a right-left division that positions him with the left. These days -- when every schoolboy is an expert on the credibility gap -- that cannot be achieved merely by a move toward the center. On the contrary, the senator needs to emphasize a different dimension entirely.

The theme that now asserts itself is activism. The country is under pressure and the Carter administration is too weak to do anything except roll with the punches. It advocates recession as a cure for inflation. It favors private giving to deal with the problem of Cambodian famine. It wants doubling up in cars to meet the energy problem It is prepared to sell out friends to soften challenges from Soviet clients.

Whatever else he may be, the senator from Massachusetts is not like that. Like his brothers, he forces problems to the surface. Like his brothers, he puts truly good minds to work on hard problems. Like his brothers, he creates, about national conditions, a sense of urgency -- even of excitement.

That is the platform he is starting to run on now. And if he can put it together he will not only beat Carter easily but also be in good posture to take the challenge that Sears hopes to mount from the Republican side.