Four years ago President Reagan and his allies in Congress pursued his program for tax and spending cuts with such single-minded zeal that they swept the government into a historic shift in direction.

Now, six weeks into Reagan's second term, so many issues have grabbed the attention of Congress and the White House that his essential themes have been blurred and the momentum of his programs slowed, raising at least a hint of the cacophony and confusion of the early days of the Carter administration.

While the budget still dominates the agenda of congressional leaders, attention already is being diverted by an imposing array of other pressing issues, ranging from farmers' credit problems to the MX missile, aid to Nicaraguan rebels and the upcoming arms control talks.

Looming not far off are such other complex and politically difficult issues as tax simplification, overhaul of farm programs and a variety of nagging matters left from previous years.

The result, in the opinion of some lawmakers, is a free-for-all climate exploited by jittery lawmakers eager to strike early blows for reelection in 1986.

This has slowed the momentum for deficit reductions, a problem compounded by the fact that Reagan, to the annoyance of his lieutenants in Congress, did little to sell his spending-cut proposals after introducing them in his budget a month ago.

Partly because of this annoyance, he dedicated his Saturday radio address to budget reduction.

"Reagan isn't in control, Tip [House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.)] isn't in control, events are in control," said a disgruntled House committee staffer.

A major problem for Reagan is the Republican-controlled Senate's struggle over farm credit financing, in which Democrats and dissident farm-state Republicans defied Reagan and the new Senate Republican leadership in approving liberalized government relief.

The week-long scramble to befriend the farmer not only held up budget work but shattered the notion of spending restraint and political discipline that Senate leaders were trying to foster to win support for controversial cuts.

It bloodied Senate Majority Leader Robert J. Dole (R-Kan.) just a month after he took over the post, strained relations between the two parties and caused rifts between the White House and panicky farm-state Republicans facing reelection in 1986.

Moreover, it beeped out a message to other needy, worthy constituencies in the country that, despite pressure for deficit reductions, Congress still heeds the "squeaky wheel," as O'Neill put it.

Reagan is likely to win the farm battle with a veto, but the Democrats seemed to win the political war. "They can't lose because their bad provisions will never become law and they can always say they tried to help the farmer," said a rueful Dole.

One Senate Republican, speaking not for attribution, drew a parallel between Carter's scattershot agenda and the proliferation of priorities at the outset of Reagan's second term. "It killed Carter, and it's not going to help Reagan," the senator said.

But Reagan continues to exert a stronger hand over Congress than Carter did, reinforced by a popularity that tends to restrain all but the most rebellious lawmakers.

And in some ways, there are advantages for Reagan in throwing all the darts at the board at once.

For instance, the arms control talks, due to start Friday in Geneva, are having the paradoxical effect of strengthening pro-defense forces on Capitol Hill, largely because of arguments that a continued show of strength at home will win Soviet concessions that could improve chances of an agreement.

Reagan has exploited that sentiment in public and private lobbying of lawmakers. Knowledgeable sources in Congress say it is paying off in increased support for the missile, which appeared near death a few months ago.

In a broader sense, it is part of the conventional wisdom of Capitol Hill that presidents, especially in their second, or "lame-duck," term, had better try for all they can get as quickly as possible before the rosy glow of their reelection fades.

So an early move on MX, for instance, may be helpful to the administration. But the White House had counted on a Republican budget accord by now, opening the track for other issues, including its tax simplification plan. And protracted budget haggling may serve to get in the way of other issues as their time comes.

There seems to be no single reason for the proliferation of issues, nor for the early startup of partisan politicking over them.

One reason seems to be Reagan's lack of a central focus for his second term; another is the arrival of deadlines set by Congress last year when it was trying to avoid tricky decisions before the elections; another is a long list of problems deferred from preceding years, ranging from civil rights to environmental controls.

Still other issues -- from the farm crisis to the Soviet Union's sudden willingness to go back to the arms control bargaining table -- arise out of circumstances largely out of the immediate control of the White House or Congress.

In many important ways, Reagan still controls the agenda on Capitol Hill as he rides high in the polls following his landslide reelection.

With the partial exception of defense spending, the budget debate is being conducted largely along the lines of his script, focusing on a new round of massive domestic spending cuts with tax increases ruled out of bounds from the start.

Even on defense, despite strong bipartisan support for big cuts in Reagan's defense spending proposals for next year, few doubt that the Pentagon will wind up faring better than, say, the Department of Education.

But the exceptions are major.

Pressure to slow Reagan's massive military buildup is greater than ever before, symbolized by the fact that Senate Republican leaders keep coming back to the idea of an across-the-board spending freeze that would include defense, an idea they would not touch in earlier years.

And the administration's refusal to cut a deal on defense has contributed significantly to Senate leaders' failure to wrap up a balanced plan in which specific program cuts, shaky on their own, could be defended as an essential part of the whole.

The administration's push for continued aid to antigovernment "contra" forces in leftist Nicaragua is another apparent exception, setting the stage for a foreign policy showdown that even many Republicans think that Reagan is bound to lose.

For all their troubles, however, Congress and Reagan will have one thing in common over the next two difficult months: escape. Reagan is planning to spend early April in California and early May in Europe. With its normal spring break not coming until early April, Congress last week scheduled an impromptu break for next week after discovering that its leaders would be in Geneva then for the arms talks.