When six loyal Republican senators gathered for breakfast with President Reagan in the White House residence last Tuesday, they gave him an unvarnished assessment of the severe political and fiscal problems confronting his administration that also was, in many ways, a message from the president's own staff.

The meeting, at which Reagan was strongly urged to take what Sen. Paul Laxalt (Nev.) called "bold and decisive" steps to bring down rapidly mounting federal budget deficits, culminated a concerted effort from inside and outside the White House to make Reagan recognize that his most cherished goals could not be reconciled.

It was an effort in which senior White House officials laid the groundwork and opened doors at the right moment, but discreetly left it to Republican congressional leaders to do the talking.

After at least three congressional meetings last week, Reagan--who had been prepared to send Congress a budget one of his aides said would have "the half life of half an hour" on Capitol Hill--showed signs of hearing the message that he must compromise to reduce huge projected deficits. It remained uncertain whether Reagan ultimately would heed the advice.

Yet the urgency of the message has helped unite, at least for now, Reagan's previously divided White House staff, instilling a fear-induced sense of cohesion among senior aides who were "at each other's throats," in the words of one, over trivial issues just a few weeks ago. "What has happened over here is that the cold, gray light of dawn is breaking," said one White House official. "The petty stuff, the friction junk, becomes unimportant when you're facing the trouble this administration is facing. We have big problems in River City. That doesn't mean we're happy, but we have to get along."

As Reagan enters the third and potentially critical year of his presidency--during which he must decide whether to seek another term--a new chapter is unfolding in his corporate style of government, one in which he prefers to have at least four chief executive officers.

A year ago at this time it was widely predicted that it would be impossible for any president to continue governing with a trio of supposedly equal top aides: chief of staff James A. Baker III, deputy chief of staff Michael K. Deaver and counselor Edwin Meese III. The forecasts were correct, but not as intended.

The trio instead has become a quartet that includes national security adviser William P. Clark. Some, who would argue that the collective leadership is even larger and more diffuse, also include presidential assistant Richard G. Darman in this top group.

But the growing number of senior staff voices, though often discordant recently, are at the moment singing a single tune. It is a warning song to the president that he must choose among his ambitious and conflicting goals of reducing taxes, massively building up the country's defenses, and stemming a flood of red ink in the federal budget.

Less then two months ago, rival camps of senior White House aides were battling over perquisites and such things as who gave the most money to United Way. Last week, by contrast, they were pulling together in pursuit of the common goal of convincing Reagan that he must make more than cosmetic cuts in his expensive defense buildup and agree to other ways--including a suggested freeze on government spending--to reduce the budget deficit.

This cohesion may last no longer than the current crisis. But for now, at least, the urgency of turning the president around has convinced Reagan's lieutenants to put aside their policy and personality differences.

This was demonstrated when Meese, who champions the most conservative views among Reagan's senior advisers, took the same position as Baker and Darman in trying to convince Reagan to compromise on defense spending. They were reinforced by Secretary of State George P. Shultz and by the insistent arguments from Reagan's GOP allies in Congress.

It also became clear to Reagan's senior aides, according to interviews last week with nearly all of them in the West Wing of the White House, that the president did not want any of them to leave or be forced out in a power struggle.

"I think there's a realization that there wasn't going to be any change," Deaver said, "so let's get with it and work it out."

Baker insisted, "I'm happy." And Meese said, "In my opinion, the system is working very well . . . ."

"The situation with Jim and Ed and me is a lot better than it has been for a long time," Deaver said. "I can't tell you why. It's just that we're not at each other's throats anymore. And I think that part of it is that everyone just got tired of it all."

The White House meetings last week, at which the president's staff and his loyalists in Congress urged Reagan to face reality, had their roots in events during December's lame-duck session of the last Congress.

At the suggestion of James Baker, the White House had invited Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) to give the president a frank assessment of budget realities for fiscal 1984. Sen. Baker, in the midst of the bitter lame-duck session, declined, saying the fiscal 1983 budget battle should be settled first.

What was scheduled instead, after the lame-duck session ended in December, was last week's Reagan breakfast with Howard Baker and five other senior Republican senators, referred to collectively by White House officials as "The Sixpack." By this time, the fiscal 1984 budget dilemma had become even more grim and urgent than when the White House congressional liaison, Kenneth Duberstein, first passed along the White House invitation to Sen. Baker.

In late December, the White House, assisted by the Treasury Department and the Office of Management and Budget, developed new, more pessimistic economic assumptions that produced a forecast of far higher federal budget deficits over the next five years than had a projection given to Reagan two months earlier.

The new deficit projections were so high, according to one administration official, that OMB Director David A. Stockman was "shocked." But the numbers were not shown immediately to the president, who spent the New Year's holiday vacationing in Palm Springs.

Reagan returned to Washington last Sunday evening, facing two meetings his staff arranged the next day. In the first, his economic advisers would show him the new forecasts. The second would be a meeting with a core group of House Republicans called by one of them, Newt Gingrich of Georgia, Reagan's "allies and scouts."

When White House aides discovered that the order of the two meetings had been reversed by a typographical scheduling error, they switched them back to make certain the president heard the bad economic news before meeting with the Republican congressmen.

The preparation, planning and scheduling was done by the White House staff. The message was carried by the congressmen. "Is there a confluence of interests?" one administration official asked rhetorically afterward. "The answer is yes--you bet."

"Some of the president's strongest supporters were volunteering to him that adjustments were necessary," said one White House official after the meeting. "These weren't liberal Democrats. Here were people Reagan felt he could trust."

Not coincidentally, the meeting was attended by Secretary of Defense Caspar W. Weinberger, considered by the White House staff to be the single greatest obstacle to a change of course by Reagan. Emphasizing both the new White House staff unity and the adversary position of Weinberger, a senior administration official said last week: "Now it's Cap and the president against the rest of them. They have some chance of winning over the president."

At the meeting with House Republicans, Reagan was given a letter written by Gingrich outlining a possible two-year spending freeze as one answer to the deficit. Reagan told the other Republicans that he wanted to hear their ideas, and the congressmen obliged.

Running through their comments was the warning that, to save his larger economic program, Reagan had to make some compromises, particularly on defense.

Early the next morning, the senators known as The Sixpack drove to the White House for their breakfast meeting, which was not announced in advance and was arranged so that the senators would not pass the television cameras clustered on the White House driveway.

The group included the three Senate leaders--Baker, Senate Finance Committee Chairman Robert J. Dole (Kan.), and Senate Budget Committee Chairman Pete V. Domenici (N.M.)--known irreverently in the White House as the "College of Cardinals."

They were joined by Laxalt, Reagan's closest ally on Capitol Hill, Senate Appropriations Committee Chairman Mark O. Hatfield (Ore.), and Senate Banking Committee Chairman Jake Garn (Utah). From experience, White House officials expected either Domenici or Dole to lead the charge.

But Garn unexpectedly took the lead and told the president he needed to find some significant savings in his defense budget. Laxalt then jumped in and said Reagan should display a bold stroke of leadership on this issue.

This approach appeared to appeal to Reagan, giving the White House staff its first clue that he indeed was listening, even if he had yet to be fully convinced.

Laxalt made a further contribution. Driving his car out of the southwest gate where reporters congregate, he uncharacteristically disclosed details of the meeting with Reagan. Rolling down his car window, Laxalt gave reporters a modified version of what Garn had said in the meeting, thereby ensuring the message would be widely publicized.

The week's events showed that the White House staff may again be functioning as it did early in the Reagan presidency and again in 1982, when the staff, after a long process of friendly persuasion, convinced Reagan to espouse a tax increase favored by both the Republican and Democratic leaderships in Congress.

That effort was so difficult, however, that Baker, Darman and other staff members were disinclined until recently to try again to persuade their sometimes stubborn president to change his mind.

"We spent weeks NOT talking about defense," recalled one official. "We weren't ready to talk about 1984."

What changed this view is the harsh fiscal reality that the White House staff now hopes will influence Ronald Reagan. This time, congressional Republicans are in the forefront of the struggle to influence Reagan, while the White House staff draws the road map.