For Jimmy Carter, news of the breakdown in the critical coal strike negotiations came with ironic timing. He was heading back to New Hampshire, almost two years to the day after winning the primary that set him surely on the path to the White House. Now, as President, he was facing perhaps the most serious test of his leadership to date.

Before he left Washington for a weekend of campaigning for other Democrats in New England, one of his aides was musing about the realities of the coal strike and some of the lessons of the Carter presidency thus far. "Many of us are continually becoming more aware of the lack of powers of the President," this person said.

And, only hours before the bargaining talks broke off early Saturday morning, a high administration official was reflecting on another aspect of the coal strike stakes for the country and for Carter's presidency. "If he pulls it off," the official said, referring to the president, "it will be a tremendous victory for him. But he has also taken a tremendous risk."

The risk was in directly placing his personal prestige on the line by bringing a national labor dispute into the White House for the first time in 14 years. After that step was taken it was the president - not his Cabinet officer, nor any arbitration board he might have appointed - who would reap the rewards of success, or bear the blame for failure.

As the bargaining sessions were drawing toward a showdown, a labor operative unconnected with this strike but with long Washington experience was worrying out loud about another aspect of the White House handling of the situation.

"I wonder if they've look ahead and weighed all the possibilities," he said. "That's what worries me, because you often get the impression that they don't seem to anticipate all the ramifications of their actions. Suppose he does order the miners back to work? Nobody I know thinks they'll obey that order. Then what? Do you call in the troops? Then What? Is there any evidence that the National Guard or the Army can operate the mines?"

The concern of a former goverment official who served two Presidents during the 1960s was different, but no less acute. He was struck by the direct challenge to the president's authority when the coal operators refused to resume negotiations inside the White House Wednesday night at Carter's request.

"If this president, as weakened as he already seems to be couldn't get those parties back together then he just might as well have abdicated, because he would be powerless forever after that," he said.

The president did succeed, of course, in getting the talks started again, but it took the threat of forceful action. Only after the companies were informed that the president was prepared to denounce them publicly the following morning did the bargaining resume. As someone close to the situation said afterward, "We were going to hit them a hard lick."

From the White House side, these kinds of criticsms are beside the point. Carter has involved himself so deeply not from lack of foresight, it is argued. Nor did he fail to appreciate the risks and problems - among them, a union virtually in a state of anarchy, and coal operators fully aware of those weaknesses.

And it isn't just the present difficult situation, with its impact on the economy and prospect of violence and disruption if a resolution is not achieved swiftly, that lies at the center of the President' concerns. It's his view that the soft coal industry represents the best hope for solving America's future energy needs. That means a stablizied industry, with modernization of mining techniques, increased production, and equity for the workers - none of which exists now.

That was the theme Carter struck when he spoke privately to both the miners and the company officials Wednesday night in the White House.

The coal industry was the single best hope for dealing with our energy needs in the future, Carter said, according to someone present. The key was to guarantee the stable, predictable production of coal.

That personal appearance bave some insights into Carter's handling of a crisis. He was calm, measured throughout. No armtwisting, no hints of threats or employment of presidential powers. No flag-waving, either.

Every president but Washington had lived inside the White House, he began. They all had faced a lot of decisions, a lot of difficulties. Fortunately most of them had operated for the good of the American people.

He believed in the collective bargaining process. It was good for the country, good for the union, good for the companies. The country's interests were at stake. People's lives were at stake. He knew of the contributions miners and coal operators have made to the United States. He knew that many of the people in the room had parents and grandparents who had been in the mines. He had been down in the mines himself in Pennsylvania and had some appreciation of what it was like to work there. He knew they understood that what they did not affected not only themselves, but everyone in the United States.

The next day the talks resumed, but in notably inauspicious circumstances. Privately, the prospects did not appear promising. Then, amid presidential reports of progress, the talks broke down again, to be resumed some hours later. It was still in the balance.

Carter was in Nashua, N.H., handling questions from high school students yesterday, when he advertently answered important questions about himself and his leadership. He was entirely aware of the risks for his presidency - and for the country - in the outcome of the present deadlock. If the negotiations fail completely, he said, it will hurt him as president. It will also hurt the country. There are hard positions that are difficult to change. And: "I can't predict success."

But he was prepared to intercede personally if necessary, he said, adding: "If I have to, I will take . . . drastic action."

At this writing, this is a story without an end, and a test of presidential power without a resolution.