The internal administration struggle over a national health insurance plan is far from over, despite recent public pledges by President Carter to support a "comprehensive" program.

The interagency debate has been going on for weeks, with advocates of broad coverage for all Americans pitted against the president's economic advisers, who say a comprehensive plan would be inflationary. They want a targeted plan that might cover, initially at least, only the poor or catastrophically ill.

In the wake of Carter's statements in Fort Worth, Tex., and at Monday's press conference that he would support a comprehensive plan that also met current budgetary restraints, his advisers are left with the delicate political question of just how much is enough to satisfy the definition of "comprehensive."

After Monday's press conference, pressed by Sen. Edward M. Kennedy (D-Mass) to back the kind of comprehensive plan Kennedy and organized labor favor, Carter gave Kennedy what the senator called an assurance that his plan would be "comprehensive and universal."

"That was the key as far as I was concerned," Kennedy said in an interview afterward. "I'm sure there will be" several issues to be settled, he said, "but in terms of moving us in the right directions, we're on the track.

But the direction of the track, and all the way stops, were still being debated yesterday.

Kennedy and labor leaders have said Americans need assurance that a health plan for all will be phased in step by step, even if it may start with some limited groups and, in the president's words Monday, "take many years" to put into effect.

There is "still some discussion" and any commitment to a firm future timetable is still "an open question," presidential domestic affairs assistant Stuart E. Eizenstat said in an interview yesterday.

"I don't think [the president] has made up his mind finally - I think he's still deliberating," said Dr. Peter Bourne, presidential health adviser.

Another health official said there is "no consensus yet," said some answers well may be missing from the health insurance "principles" the president said he will complete "within the next few days."

But Max Fein, director of the Committee for National Health Insurance, a labor-backed lobby, said the "details will be what make or break" the president's plan.

"Comprehensive to my mind means first day, first dollar health coverage for everyone," Fein maintained. "We haven't got assurances yet on the degree of federal controls, federal regulation, prospective budgeting" - tightly controlled advance budgeting for doctors and hospitals - "and other things."

What people like Fein will watch for in the president's principles will be such details. But they think they may not see them until the president sends Congress the more detailed "plan" he has promised for later this year.

And they think they may not really know the president's mind until he sends Congress an actual bill, probably next year.

No one expects congressional action before 1980, which means a new health plan could not start before fiscal 1983.

This means the debate over how many Americans need more health insurance, how much can the country afford, and how costs can be controlled will continue.