Jimmy Carter is entering a period of his presidency when the public attitude toward him, which until now has seemed tentative and ambivalent, is likely to begin to solidify one way or the other.

The president who promised much and delivered little in his first year is now facing a series of key tests on issues he has declared to be of utmost importance of him and the country.

The first of those tests will come Tuesday with the Senate's vote on the second Panama Canal treaty. White House aides remain confident that the administration will prevail on the treaty issue. But the White House's handling of the first treaty, its acceptance of the "DeConcini reservation" in the face of warnings from the Panamanian government, has already contributed further to the image of a president desperate for a victory.

Then there is Carter's cherished energy program. This week marks the first anniversary of the president's "moral equivalent of war" speech to the nation. Six months ago, it was the energy program that presidential aides looked to for the first major breakthrough in dealing with Congress.

The breakthrough still has not come. But Carter's personal intervention in the energy negotiations last week was a recognition that the stalemate cannot go in indefinitely, that, in the words of press secretary Jody Powell, "time is running out on everybody," president and Congress alike.

Other tests are piling up behind Panama and energy. The president has now committed his administration to a major campaign against inflation.

On Friday, the White House reiterated Carter's determination to win congressional approval of his tax cut and tax "reform" package.

And sooner or later - and not too much later - the president's repeated claim of "good progress" in the strategic arms limitation talks (SALT) with the Soviet Union is going to have to be demonstrated or forgotten.

The next few months are very important to the Carter presidency," Richard Moe, Vice President Mondale's chief of staff, observed last week. "Obviously the Panama vote, obviously the energy package, obviously SALT. On inflation, the first few months probably will tell us if it [the administration program] is going to work."

Moe added: "Whatever attitudes the public forms toward an administration in the first 18 months or so will probably set the attitude for the remainder of the term."

Patrick Caddell, the White House's political pollster, agrees that "at some point in the middle of a term people begin making some fairly hard judgements about a president."

Caddell says he believes that the public generally is somewhat uninterested in the Washington political scene now and that this may have contributed to its less-than-certain attitude toward Carter.

"People have not necessarily reached hard judgements on him," Caddell said.

But hard judgements will be made, and they are likely to center not so much on Carter's personal qualities - Caddell and his friends in the White House insist that the president remains personally popular and is trusted by the public - but by how he is seen handling his job. And on that score, his steady decline in the polls, reaching a low of 46 per cent approval in a New York Times - CBS News poll last week, cannot be comforting.

Where the White House does find comfort is in the fervent belief of presidential aides that Carter is held in higher esteem in the country as a whole than he is in Washington political circles.

But the White House is possibly overly optimistic in that assessment, at least as it applies to the president's standing among his fellow Democratic politicans around the country.

In Washington last week, two Democratic Senate candidates - Charles D. (Pug) Ravenel of South Carolina and Joe Christie of Texas - said Carter was only "about 50-50" in helping or hurting them in their races. Both are pro-Carter Democrats from states the president carried in 1976.

On a recent swing through the Midwest, Washington Post national political correspondent David S. Broder found an even cooler attitude toward Carter.

"They have done almost everything imaginable to make it tough for us," said one farm state governor of the administration in Washington.

And even 1,000 miles away from Washington, the Carter White House staff, so much criticized here, is a subject of discontent.

"I think I have a good staff in this state and they haven't gotten me in trouble here," said that governor, who is on the ballot this year. "But don't think there are more than two of them I'd take to Washington if I were president. Carter took everybody, and he hasn't acted like he needs anybody else."

Inside the White House, there is a general, if somewhat reluctant, recognition that the president has an image problem. It is an image of a tentative, some would say weak, chief executive, uncertain in his dealings with Congress and foreign governments.

More and more, comparisons are being made with the man Carter defeated, Gerald R. Ford, who ended his two years as president perceived as an honest, well-meaning man who was not quite up to the job.

"I don't think he's [Carter] perceived as a leader now, and steps need to be taken to correct that," one Carter aide said. "He may never talk like a leader unless he takes lessons."

Powell, who has deftly defended the Carter record for the last 15 months, concedes that the image problem exists. But he argues, as he has almost from the first day of the administration, that it is the inevitable result of the president's decision to "take on" some of the most difficult problems facing the country.

"There is no way to deal with it in the short run without changing our minds about what we came here to do," Powell said. "To deal with these things, it's going to be tough as hell."

But, in fact, the White House is trying to do something about it, now, before it is too late for the 1980 elections. It was no coincidence that the president chose the spring of his second year in office to summon his entire Cabinet, his senior White House advisers, the vice president and Democratic Party chief John White to Camp David.

They are all meeting today and tomorrow in the Catoctin Mountains, threshing out what has gone well, and, more importantly, what has gone wrong since January 1977.

Nor is it a concidence that Carter's chief political adviser, Hamilton Jordan, is now implementing changes in the White House staff. The details remain vague, but new people will be brought into the White House, and operations that are considered weak - the public liaison and counsel's offices are most frequently mentioned - will be, aides say "shored up."

When the administration comes down from the mountaintop, it will presumably be with a renewed determination to gain for the president some of the victories that have eluded him so far.

"If we do well and break through on some of these things in the next few months, we'll do well in 1980 and it won't make any difference who runs against him," said Moe, who admits to being an eternal optimist in such matters.

And if they do not do well in the coming tests, Powell said, "we'll have a fairly seriousproblem."