When his campaign reached low ebb last month, Sen. Edward Kennedy invited Rep. Morris Udall to explore an unusual political proposition -- the feasibility of Udall's supplanting Kennedy, with Kennedy's blessing, as the principal Democratic challenger to President Carter.

The fact that both men came to the conclusion that party rules and political circumstance would make such a switch impossible in no way diminishes the significance of that meeting. It illustrates at one and the same time why Kennedy, in adversity, has earned the admiration of even his detractors, why the Wisconsin primary results are a truer indication of Kennedy's presidential chances than the results of New York and Connecticut and what might be a constructive and useful role for the senior senator from Massachusetts to play for the balance of this political year.

Kennedy's days as a competitor for presidential power are over, but his role as a force for political change need not be.

There has been a certain nobility to the last two months of the Kennedy campaign.

The candidate has faced defeat with dogged determination. He has met attack and innuendo, fair criticism and abusive assault alike with humor, grace and more forbearance than could be expected of a mere mortal. wHe has stood his ground, battling for his beliefs long after it seemed prudent, long after any real personal gain seemed possible and long after many of his closest advisers urged him to cease and avoid further embarrassment.

His willingness to remove himself from the contest in favor of someone he believed might more effectively challenge Carter's policies and presidency is a reflection of a selflessness that was never evident when the Kennedy family was riding high. But that very gesture contained within it a tacit admission that his candidacy might be carrying too much excess baggage -- in the form of negative public perceptions of his character, abilities and private life -- to permit a successful assault on the presidency.

That admission is, of course, simply a realistic reading of revealed truth as expressed in both the polls and early primaries. For Kennedy received support in New York and Connecticut long after the public had soured on Carter's economic and foreign policies and only when Kennedy appeared no longer to be a serious presidential contender.

The Democratic voting public seems quite willing on occasion to use and vote for Kennedy as a vehicle for venting its displeasure with Carter. It has shown no comparative inclination to replace Carter with Kennedy.

As the Wisconsin primary showed, the personality issues that were temporarily submerged under a wave of anti-Carter hostility in New York can surface again at any time and would almost certainly surface again in any general election contest -- to the detriment of his party and the very ideas Kennedy and his supporters hold most dear.

Kennedy may be, as his meeting with Udall seems to indicate he knew, just too encumbered by negative public perceptions to be a viable Democratic nominee or American president.

What then for Kennedy?

It seems to this observer that Kennedy should follow the implied conclusions of both his meeting with Udall and the electorate's voting behavior -- to become a force rather than a candidate.

He should consciously and overtly offer himself in the ensuing primaries, not as a candidate for president, but as a vehicle for a public referendum on the Carter presidency and especially its economic policies, which have, in effect if not intent, served to increse inflation, create recession, diminish the effectiveness of government and make those least capable of defending themselves suffer. He should confront Carter with what is indeed Carter's real choice -- to change his economic policies or face division within the Democratic Party and defeat in November.

In such a role, Kennedy could serve as a force for both challenge and conciliation:

Should Carter alter his economic policies so that they were more effective and compassionate, Kennedy could serve to bring contending factions together for the November campaign.

Should on the other hand, Carter persist in his course, Kennedy's continued presence on primary ballots would offer the public an opportunity for judgment.

Should, in turn, that judgment become increasingly negative -- as it well might in Kennedy's targets of opportunity in Pennsylvania, Michigan, New Jersey, Oregon, Ohio, California and the Rocky Mountain states -- Kennedy might broker the convention for another nominee.

Should the convention persist in nominating Carter despite divisions over his policies -- in a manner not dissimilar to the 1968 Democratic Convention -- Kennedy could choose to give or withhold his support pending the nature of Carter's general opposition and his own perceptions of the national interest.

The success of such a strategy depends on Kennedy's willingness to make it overt, for there is a large group of Democrats who would like to vote against Carter but will not do so as long as they believe their vote might nominate Kennedy.

This role is not what Kennedy had in mind when he set out to run for president. On the basis of his performance in the last month and a half, he surely deserves better. But it is an importnat and useful role to play, and it falls well within the scope of what his brother Robert believed when he described politics as "an honorable profession."