Sen. Edward M. Kennedy is beaming out at unsmiling presidential aides from three television sets in Jody Powell's office -- beaming despite (or, perhaps, because of) the crush of cameras and technicians jostling him as they fight to record the fact that he is going to the theater.

It is a grin familiar to the unsmiling Carter aides, because it is not unlike the grin that became the trademark of their boss, back when he used to bask in cameras and cables as he made network news by draining the fish pond in Plains in the summer of '76.

"I think it will be a hard-fought battle, both the nomination and the election," Kennedy is saying on TV, on a newsclip that has been videotaped by Army Signal Corps personnel aides for easy reviewing by the Carter aides.

The senator may have slipped inadvertently out of the traditional veil of the subjunctive, but President Carters advisers figure Kennedy actually meant what he accidentally said.

Kennedy's plan is that he will be running for president, the Carter advisers have concluded.

Out on the white-columned portico just outside the windows of the Oval Office, President Carter pauses for a minute to chat about the news that has dominated the newspapers and the networks this past week. "I think we're better off with Kennedy finally being out in the open," the president says, according to one senior adviser who was there. Now, Carter and his advisers have concluded, Kennedy will begin to be judged in comparison with Carter on the issues, not just on image and charisma. And on the issues, Carter believes, he holds a commanding lead over Kennedy.

But throughout Carter's White House and throughout Carter's campaign headquarters -- and in fact throughout the Congress that clearly is not his and the bureaucracy that only technically is his -- there are many who do not agree that Jimmy Carter is better off because of the events of this very political, very Washington week.

Within the White House, the apparent reality of a Kennedy challenge has left a number of mid-level loyalists shaken and wondering if all is not lost already -- and privately, some of the president's most senior advisers sound every bit as dispirited.

"It all seems to be slipping away from us," says one Carter aide. "I think it might be beyond salvaging now. It's coming apart awfully fast."

In the week that Ted Kennedy brought his presidential aspirations out of the closet, presidential advisers Hamilton Jordan and Jody Powell meet several times with campaing director Tim Kraft at the White House to discuss how it all affects the Carter-Mondale timetable and strategy.

"There weren't any bright insights," Kraft says after one meeting. "All it means is that Kennedy has upped the ante a little -- but there's still no need for us to declare Kennedy's candidacy for him."

They conclude that one reason Kennedy spoke out now is to give encouragement to the draft-Kennedy movement in Florida that is gearing up for the nonprimary, nonbinding, nonsensical straw vote that will be held in Democratic and Republican conventions next month. It is Florida's political slapstick effort at getting national attention by running the first beauty contest of the 1980 season, even if it means doing it before Halloween of 1979.

In 1976, Powell remembers, Carter won a similar Florida preseason caucus beating one Milton Shapp, who was the only other candidate who bothered to be there, "and after we won we had a hard time even getting the wire services to move the story."

On Wednesday, the Carter advisers dispatched campaign counsel Tim Smith to Florida to assess the operation in that state and do some press interviews. He leaves immediately.

Meanwhile, Hamilton Jordan says, "it's not in our interest to analyze or type all of Teddy Kennedy's motivations. We've got to worry about getting energy and SALT passed in Congress."

That is Carter's feeling and it has become Jordan's line.It is not shared by all of the president's most senior advisers, however. One such adviser maintains that a failure to get the energy package or SALT approved on Capitol Hill will hurt the president further, but approval of the measures will not be of great benefit to Carter in his effort to put down the Kennedy rebellion. These measures will not really affect people's lives by the time they must heed the call of George Gallup or the first caucus gavel in Iowa Jan. 21. They will just indicate whether Carter is able to exert leadership effectively on Capitol Hill.

Sixteen blocks from the White House, up at the eastern establishment end of Pennsylvania Avenue, Ted Kennedy spent much of the week telling people that he had come to reassess his noncandidacy because of his concern for the state of the economy -- and because of the concerns of Democratic colleagues on Capitol Hill who are urging him to run. Sources close to Kennedy elaborated that 11 of the 22 Democratic senators seeking reelection (two more are retiring) have urged him to run, fearing that they might go down with Carter at the head of the ticket.

But a Washington Post survey of the 22 senators shows only a fraction of that number who are willing to admit that they did indeed urge Kennedy to challenge Carter. George McGovern of South Dakota says he did and so does John Durkin of New Hampshire. Most of the others say flatly that they did not urge Kennedy to run -- including Kennedy's close friend, John Culber of Iowa, who said through a spokesman that he felt it was just too personal a thing, what with the assassinations of two Kennedy brothers and other family problems for him to try to interfere.

Even though they say they did not go to Kennedy and urge him to run however, almost all of the 22 Democrats say they think the chances of Democratic candidates nationwide will be much better if Kennedy is the party nominee instead of Carter.

It all comes down to turnout and apathy. "I think Ted would pull out a larger turnout nationally among blacks and labor people and others," says Gary Hart of Colorado, in comments echoed by many other senators. "They represent the traditional Democratic constituency -- and that could make a difference in close races. The biggest problem with President Carter is apathy."

Some senators and representatives from southern and border states said they would do better in their own states with Carter at the head of the ticket. And some others agreed with a midwestern senator uho said: "Kennedy increases the interest factor, but he also increases the anti-factor and the hate factor. So I can't tell whether it's a net plus or net minus to have Kennedy instead of Carter."

In the cloakrooms and corridors of the Senate and the House, members of Congress discuss quietly among themselves whether it will come to a showdown, with a delegation of highpowered Democrats journeying from Capitol Hill to 1600 Pennsylvania Avenue to tell a sitting president (as Barry Goldwater and Hugh Scott told Nixon) that for the good of his party he must step aside -- in this case, withdraw from the 1980 campaign.

Interviews with about three dozen members of Congress find a few who can envision such a scene. But most say it is unlikely, and virtually all of them now believe that Carter's response would be merely to dig the trench deeper and get on with the battle.

Many shared the view of Rep. Leon Panetta (D-Calif.), who said, "Presidential power is still there in projects and money -- and nobody is going to tip the scale before he has to."

There is, in many corners of the Carter White House, an air of resignation and even defeat. It is an air that is premature, with the first presidential primary still five months away and the wars of politics and policy still to be fought, yet it is an air that persists.

The feeling of frustration and despair exists because the Carter loyalists figure that in recent months Jimmy Carter has given it his very best shot -- and things have only gotten worse. Part of it was his own doing, but a crucial part of it was not.

"I can't help but fell that maybe Jimmy Carter is just star-crossed,' says one presidential adviser.

Consider what has happened since the president resolved, back in July, to make one last, all-out effort to rebuild his presidency and reshape the record on which he hopes to run.

First was a new sweep in the name of leadership. He gave a forceful speech to the nation one day, and torpedoed any good that it did the next with a heavy-handed, mass-resignation "Cabinet shakeup. That was his own undoing.

But then came Andrew Young, a long-tolerated stormcloud, suddenly gaining force as Hurricane Andy and veering into a new area -- Mideast policy -- wreaking havoc there, and then leaving in his wake, a politically costly debris of black and Jewish relations.

Next, Hamilton Jordan, newly annointed as chief of staff, found himself called to task for the adolescent nocturnal indulgences of his too-recent past. His susceptibility to the New York strain of disco fever plunged him into a cocaine investigation that -- Justified or not -- shattered his (and Carter's) reconstitued image of leadership.

The there was the Soviet brigade. It has been in Cuba for years, U.S. intelligence now believes, but it was Carter's misfortune to have it discovered on his watch. And worse yet, the only action that Soviet brigade may record during its stay in Cuba is the sinking of Carter's SALT pact.

So it was that all of those incidents -- plus the statements by Kennedy -- came together last week in a quickie Associated Press/NBC poll that showed Carter at the lowest point of any president in nearly three decades, with only 19 percent rating him "excellent or good," 49 percent giving him "only fair," and 30 rating him "poor," and a bare 2 percent "not sure."

"Those things weren't the president's fault, but they hurt him just when he needed a break," says one Carter aide. "Sometimes I wonder if he's ever going to get one."

Tim Kraft, he of the Pancho Villa moustache and the politically necessary eternal optimism, recognizes that luck plays a big part in politics and that lately all of Carter's luck has been bad.

"The American people hold their president responsible for everything from the grocery bill to the weather," Kraft said after his last strategy session of the week on the emerging Kennedy-Carter contest. "And when it's going bad, it can be really bad.

"But we have to remember 1976. We had a lot of lucky breaks then, the fact that certain people stayed out of certain early primaries and left the field open for us, and the fact that we had money available when we needed it most and the other guys did not.

"So we got our share of lucky breaks then -- and we're going to get our share of lucky breaks now."