Imagine the spectacle of an incumbent Democratic president squaring off in Des Moines, Iowa, against the heir to the Kennedy mantle and the futuristic governor of California in a free-for-all battle for the White House.

Imagine is all anyone can do, of course, because it never happened. We all know that plans for the great debate, scheduled for last night, collapsed after President Carter announced he would not participate because of the press of international crises.

But -- pretend they did debate. Sen Edward H. Kennedy and Gov. Edmund G. (Jerry) Brown, Jr. got the exposure they wanted, and the public presumably gained some insight into the differences between them and the president which motivated this assault on Carter by fellow Democrats.

In a media black hole such as this nonevent, the commentators are denied their judgments on personal performances: Did Kennedy bumble his words? Did Carter smile too much and sound indecisive? Did Brown's rhetoric rocket too far beyond this galaxy? That we can't say.

But we can imagine how it would have gone. This instant replay is based on and uses the three candidates' actual statements in recent months -- their more forceful, articulate and representative statements -- made in various meetings, interviews, written statements and other public forums.

Brown repeated his three-point slogan: "Protect the earth, serve the people, explore the universe."

Kennedy said, "For many months, we have been sinking into crisis, yet we have no clear summons from the center of power . . . I have a different view of the highest office, in the land -- a view of a forceful, effective presidency . . ."

Carter spoke of a national crisis of confidence but said the Iranian crisis has demonstrated that "contrary to our basic nature, Americans can also be very patient, for patience is the better part of valor."

Under the umbrella of Democratic Party tradition of course, the three share vast similarities. It was in the interests of the challengers last night to heighten the differences, in order to justify their campaigns. The issue of leadership quality served that purpose.

Kennedy, for instance assailed Carter's record even in areas where they had been in virtual agreement, such as civil rights, or tax reform, on the grounds that the president -- the Washington outsider -- had been inept or neglectful in getting results on those issues.

Kennedy suggested on the other hand that he possessed the solid Capitol Hill experience, the knack for building coalitions the political savvy to get things done.

"We cannot let the 1980s begin," Kennedy said, "with nobody in charge . . . How dar anyone tell us we must lower the horizons for our future? The sounds we hear from the White House are not the sounds of leadership but the sounds of uncertainty and retreat."

Indeed, woven through their rhetoric were indications of significant, though subtle, differences in their basic perceptions of the American people, the role of government and its leaders, the realities of the coming decade.

In general Brown had an easier time distancing himself from the president than did Kennedy, who until recently had developed a Senate record which harmonized with the Carter administration on most issues.

Brown, in fact, charged that Kennedy had become "a me-too candidate . . . a carbon copy of Carter on virtually every issue."

He offered himself as an unconventional alternative for the 1980s, to replace "the decaying myths that paralyze our nation," a depature from the "debased language of politics" used by the others, which he said inspires mostly cynicism.

In terms of position on the traditional political spectrium, Brown had the other two surrounded. He held the right as a tightwad on government spending, clung to his proposal for a constitutional amendment to balance the federal budget, and noted his record of fiscal responsibility in California.

But he was way over on the left in his support for homosexual rights, appointments of blacks, women, and other minorities, and his opposition to nuclear power of any kind.

Any at the same time he emphasized the new limits of government, he envisioned a major role for government not only in the engry field but in mass transit and the development of new technologies.

Brown denied charges that he was a political opportunist, or inconsistent in his unconventional synthesis of ideas. It is what he called the "canoe theory" of politics: "You paddle a little on the right, and then a little on the left, and that's how you move straight ahead."

While Brown was tying a rhetorical bow with the extreme ends of the political spectrium, Kennedy, staunchly reiterated his liberal ideals -- an activist government, "the party's responsibility to speak out for the powerless in society" -- and he criticized Carter for his cuts in social programs.

"There are times," Kennedy maintained, "when a party must sail against the wind."

On the other hand, Kennedy indicated he was not totally insensitive to the "big spender" label that critics have pinned on him. After all, he said, his differences with Carter amount to $4 billion or $5 billion out of a $530 billion budget.

Moreover, the issues on which he has spent the most time in recent years, he noted, were not spending issues. They were, departures from traditonal liberalism, such as major legislation to deregulate the trucking and airlines industries and revision of the criminal code, with a tough anticrime stance.

Kennedy scoffed at Carter's talk of a national "malaise" and said the American people are "willing, even anxious, to be on the march again."

Carter contrasted the limits of today with the supercharged '60s: "We have a keener appreciation of limits now, the limits of government, limits of the use of military power abroad, the limits of manipulating without harm to ourselves a delicate and balanced natural environment."

Carter added that Kennedy has failed in his own leadership, in the Senate. For example, he has failed to deliver on his most cherished legislative goal -- his national health insurance proposal, Carter said.

Kennedy countered that he had worked hard building national support for the program under hostile Republican presidents and in 1977 was looking forward to seeing it through at last with the crucial help of a supportive Democratic president. But Carter, he charged, had broken his campaign promise to pursue the kind of sweeping plan Kennedy considered essential.

The three men came to the nondebate with their relative positions in the public mind radically altered from those of, say six months ago. Back then, Carter was under attack from all sides as "incompetent," and his public approval rating was in the cellar. Brown seemed a gathering threat. And Kennedy -- all he had to do was nod, it seemed, and the nomination was his.

By last night, the question was whether Carter could hold onto his sparkling lead in the polls, whether Kennedy could recover from the verbal pratfalls of his early weeks as a candidate, and from the lingering legacy of Chappaquiddick and whether Brown could raise enough money to stay in the race even until the spring thaw.

Thus, with less and less to lose, Kennedy and Brown both unsheathed their knives on the president's foreign affairs policies, and repeated their criticism of his "weak" policy and his decision to sharply reduce grain sales to the Soviet Union.

In perhaps his most controversial policy stand, Kennedy denounced Carter's removal this year of controls on oil and gas prices and called them "an economic disaster."

That program "is a regressive plan that imposes the heaviest burdens on those who can least afford them."

He accused the president of "surrendering" to the oil companies and criticized Carter for failing to get a "windfall profits" tax passed before he moved toward decontrol. "Any farmer knows you don't give the horse the sugar and then try to get him into the barn."

The president dismissed this particular criticism as "a lot of baloney."

Brown drew the battle lines even more distinctly. He accused both Carter and Kennedy of being soft on big oil companies and proposed a stronger government role than either rival.

He called for gasoline rationing and said he would create a national energy corporation to "develop and manage our own oil on public lands," prohibit the importing of foreign oil except through the federal government and seek authority to appoint public respresentatives to the multinational oil companies to "represent the people."

On defense, Brown opposed any increase in spending and development of the MX missile. Kennedy said he was committed to a first-rate defense posture but one defined in terms of capacity, not how much was spent. He supported funds for development of the MX missile but said it should not be deployed until all alternatives were examined.

Carter, shocked at recent Soviet aggression, said he supported at least a 4 1/2 percent real increase in defense spending in each of the next five years and he supported development of the MX missile.

On abortion, Brown supported full government funding of Medicaid abortions. Kennedy said he personally opposes abortion, and has voted against federal funding of abortion on demand, but supported public funding for the poor.

Carter, explaining his opposition to federal funding of abortions for poor women, said, "There are many things in life that are not fair, that wealthy people can afford and poor people can't."

On the economy, Kennedy said that mismanagement by the Carter administration was a major factor in his decision to run for the presidency. He faulted Carter for slowness in reacting to problems and for a lack of authoritative personal leadership in making voluntary wage and price guidelines work.

Brown, calling for a constitutional amendment to balance the federal budget, said "there is no 'managerial' solution to inflation. It will take a political decision by the people."

He also proposed a seven-point formula for a sweeping "new economic order," including an agreement among oil cartel countries and the United States to hold oil prices constant for five years.