Jimmy Carter was already back at the White House today when Sen. Edward M. Kenndy neared the end of a speech on national health insurance to a workship at the Democrats' midterm conference here.

Glasses perched on the end of his nose, fist pounding the podium, Kennedy told the overflow crowd in the convention hall auditorium where he stood, where they knew he had always stood.

"I want every delegate to this convention to understand," he declared, "that as long as I have a vote in the United States Senate it will be for the Democratic Party platform plan that will provide decent health care across this country, North and South, East and West, for all Americans as a matter of right and not of privilege."

The crowd was on its feet instantly, cheering. Had the president been here, it would not have been a comforing moment. For the applause Kennedy won today from his fellow Democrats came easily, before, during and after his speech, at every opportunity.

At a convention notable so far for tedium and lifelessness, the Massachusetts senator proved again that, whatever his personal intentions and ambitions, he poses a challenge to Carter in terms both of style and of, increasingly, the substance of Democratic policy.

As for style, the contrast with the tepid reaction to the president's keynote address to the convention Friday night could not have been more dramatic. Kennedy generated a spark of life at this gathering of Democrats, something Carter, with all the accouterments of the presidency and a $65,000 movie to introduce him, did not.

And as for substance, Kennedy made clear in his speech that his differences with the White House over how soon the country can afford national health insurance could easily turn into much more widespread opposition to the president's anti-inflation policies.

Ken nedy's depiction of social inequities worsened by slashes in the federal budget invoked almost the entire litany of Democrat interest groups, implicitly warning Carter of the political risks he runs in offending them deeply.

When the cheers came, presidential assistant Stuart Eizenstat, another member of the health insurance discussion panel, stared out at the crowd impassively.

The health insurance session was by far the best attended of the 24 convention workshops today, largely because it held the prospect of a clash over the issue between Kennedy and administration spokesmen Eizenstat and Health, Education and Welfare Secretary Joseph A. Califano Jr.

That clash never really occurred. Before arrival in Memphis, a deal was struck in which Kennedy promised White House officials not to advocate a vote on a call for enactment of a national health insurance plan next year. The president favors a much slower, phased-in approach.

Kennedy kept his word on that. But as for other administration domestic policies, he gave the White House little cause for comfort.

The undercurrent of dissent and sisatisfaction by liveral Democrats over the "very tight" budget Carter has promised for next year has been a central theme of this convention. But it has been an unfocused and leaderless dissent. Kennedy provided the focus and told the president, in effect, that someday he might provide the leadership.

"We meet," he said, "at a time of caution and uncertainty in the land. The hopes and dreams of milllions of citizens are riding on our leadership. Sometimes a party must sail against the wind. We cannot afford to drift or lie at anchor. We cannot heed the call of those who say it is time to furl the sail."

Carter knows well of the party dissent and seems supremely confident that he will prevail. In his soft Georgia drawl, he told an earlier workshop on the economy and inflation:

"My own assessment in history, my own political fortunes will be determined by Americans' judgment when I make decisions. For instance, in the 1980 budget -- had Jimmy Carter been fair? Has he been conversant with and sensitive to the social needs of our people?"

This afternoon Kennedy replied in his fiery Boston Irish baritone that he will be among those watching and judging.