A few days ago, an aide to Vice President Mondale was speculating about the many difficult decisions that are crowding in on President Carter and what those decisions may tell about the direction of the Carter administration.

The President, he noted, has carefully and skillfully nurtured his public support, building what is known in the trade as his store of "political capital."

"There is no question that he is willing to spend that political capital to get what he wants," the Mondale aide said. "The interesting question is what will he choose to spend it on."

Late last week, Jimmy Carter began providing some answers to that most interesting question, and he will continue to provide answers all through the coming week.

The President is now in the midst of an extraordinary nine days of activit, dominating the national television airways perhaps more than any President before him. He has already announced or will shortly announce some of the major decisions he has made inside the cloistered walls of the White House, and with them set the tone and agenda for much of the political debate that will follow in the months ahead.

They are decisions that have to do with taxes and inflation, energy and water projects. They are decisions that will affect, perhaps deeply, the American economy, American lifestyles and Carter's own tenuous relationship with the Congress and the interest groups that seek his favor.

In a sense, this is a time of transition in the Carter presidency itself, a move away from the easy days of symbolism to the hard business of choosing among competing interests.

White House officials are convinced that Carter entered this most crucial week of his presidency in as strong a political position as could reasonably be expected. Through three months of fireside chats, town meetings and assorted other examples of Carter's justfolks style of leadership, they have tracked, in their own polls and others, the President's astounding rise in standing with the public.

This goes beyond mere approval of his performance to date to what White House press secretary Jody Powell calls "the single most crucial aspect for any President" -- whether the people trust him and believe he is honest and sincere. In this, Powell said, Carter pollster Patrick Caddell has found a dramatic drop in the number of people with negative feelings about the President. The percentage of those who have placed their trust in Jimmy Carter has treaked from the 51 per cent who voted for him into the 70s and 80s.

"You can get re-elected and you can even get legislation enacted even if people don't trust you," Powell said. "We've seen that. But you can't really lead . . . We are likely now to be in as strong a position as we ever will be."

The President has already revealed much about his own priorities for the coming months. Above all else, he is staking his hopes and prestige on achieving what no other President has --policy. The subject of two nationally televised presidential addresses within three days, energy will be the dominant topic of conversation this week. It is for his energy policy, White House aides say, that Carter will spend his hoard of political capital.

The prospect of a long, divisive battle to achieve the energy policy has already played a role in some of Carter's other decisions of this week.

When he announced last Thursday his abandonment of the $50 tax rebate portion of his economic stimulus package, the President stressed the economic rationale behind the decision. With the economy showing signs of improving, he said, the more than $11 billion in tax rebates seemed suddenly unnecessary.

But Carter and his aides also readily conceded there were political aspects to the decision, among them a calculation of whether the rebate was worth the tough Senate battle it would take to gain it. With the energy message due the same week of the scheduled Senate vote, it was decided it was not. Carter pulled back preferring to fight another day.

The President with the reputation as a stubborn, uncompromising chief executive will announce on Monday another series of decisions that are fraught with compromise. Many of the 30 water projects he once described as wasteful and targeted for extinction will be allowed to continue with at least partial funding.

How much the impending struggle over energy figured in this is not clear, but this much is: men like Rep. Morris K. Udall (D-Ariz.), whose cooperation is vital in gaining the energy policy and in establishing tough new criteria for future water projects, will be placated. The $1.8 billion Central Arizona Project, which, according to one White House aide, Carter objected to right up to Friday morning when he made his final decisions, will receive continued funding with some modifications in its design.

The President's "week," with its accompanying saturation media cover age, actually began last Thursday with an unscheduled and unexpected development. Carter strode into the White House press briefing room that afternoon to announce his abandonment of the tax rebate, the first, clear-cut turnabout of his young administration.

Thursday night, although not technically part of the White-House-or chestrated media blitz of the week, the President received one final boost in the image department. He was the subject of an hour-long NBC television special entitled "A Day in the Life of Jimmy Carter." It was an intimate and folksy glimpse of the President at work, of immense value to a man who has promised to confront his constituents with some unpleasant news about energy in just a few days.

Friday morning, Carter was back on national television with his fifth press conference since taking office. The purpose was to announce a relatvely mild, completely voluntary package of anti-inflation measures, but the questions that followed were dominated by the tax rebate decision and the upcoming energy message.

But the real push will come in the next five days, beginning with an address to the nation on energy Monday night.

In keeping with the importance the White House places on the energy issue, Carter will devote two separate speeches to the subject this week. The first, on Monday night, has been dubbed as "the sky is falling" speech, in which the President will grimly set out the extent of the nation's energy problem.

The Monday speech has also been described as a second "fireside chat," but that is a misnomer. It will be delivered from the President's place of business, the Oval Office, not his residence. Its tone, far from chatty, will be businesslike and presidential.

The nation will be given one day to digest the bad nes before Carter makes his first official journey to Capitol Hill. In a speech to a joint session of Congress on Wednesday night, the President will propose his solutions to the energy problems he will detail Monday night.

Another day for digestion will follow until Friday, when Carter will cap the week with yet another press conference, this one certain to be dominated by energy.

Press secretary Powell was concerned enough about the extent of this media blitz to consider shifting next Friday's scheduled press conference to early the following week. But late last week it was decided to go ahead with the week as originally planned.

"It's a calculated risk," Powell said, "It's an awful lot of exposure."

Powell expresses satisfaction with what has come before this week of decisions. About to deliver an energy message he has predicted will severely test his public support, the President is riding high in the polls, presumably hoarding a vault full of precious political capital.

"Now we've got to put that to the test and say, 'This is the problem as I see it,'" Powell said. "'This is what we have to do.'"