If the Reagan administration were a baseball team, it would drive its fans crazy. Every spring, it slumps. Spring training games don't count in the final standings in either baseball or politics, but the pattern is a puzzlement.

A year ago, President Reagan was having problems over the budget, aid to the contras in Nicaragua and the visit to the German war cemetery in Bitburg. The popular explanation for the "wobble" in the administration was that the rookies at the White House, under freshly installed chief of staff Donald T. Regan, were still learning to play their positions.

This year, Don Regan is so firmly in charge that he was parodied at the Gridiron Dinner the other night as the "Rambo" of the administration and Washington. But the spring stumbles continue. Aid to the contras is rejected in the House; both House and Senate reject Reagan's budget and prepare to rewrite it in substantially different form.

It is becoming clear that the difficulty lies not in the management of the president's agenda but in the agenda itself. Reagan has one idea of the nation's top needs; Congress has a very different idea.

The difference goes back to the 1984 election. That was Reagan's opportunity to renew the mandate he won in 1980. In his first election, he made his priorities plain: reducing taxes, scaling back the federal government's domestic role and shifting massive resources into defense. Congress, responding to the clearly expressed public will, did all three of those things in 1981.

In 1984, by contrast, the president's massive victory was based more on the accomplishments of the past than on the agenda for the future. His second-term plans were no more specific than the boast that, "You ain't seen nothin' yet." Congot regard that as a commitment it is required to fulfill.

Lacking a policy mandate endorsed by the voters, Reagan at 75 is guiding himself by principles that have been embedded in his philosophy for decve opposition to communist regimes, even if they are lodged in such third-rank countries as Nicaragua. Another is a gut conviction that the bigger the military budget the better. And a third is a deeply felt desire to keep reducing tax rates for individuals.

Reagan believes that these views are shared by the nation, which twice overwhelmingly elected him. But the public opinion polls and the judgment of the politicians on Capitol Hill suggest otherwise.

The tide turned against the president on military spending more than a year ago, and he has never had public opinion on his side when it comes to any form of intervention in Nicaragua. On taxes, while the public is opposed to higher rates, substantial majorities would forgo further cuts in order to reduce the federal deficit.

In these three areas, at least, the evidence suggests that Reagan is swimming upstream against strong political currents. Congress is closer to the national consensus in pressing for lowered deficits through restraint on defense and repair of the revenue base. And its skepticism on aid to the contras is certainly shared by most voters.

Yet Reagan's power and determination are far greater than those of his opponents in Congress. They cannot simply ignore his agenda and proceed on their own, because they acknowledge that he has greater political resources than they can command.

This kind of impasse does not yield easily to repair, for it is rooted in the very election that created this particular balance of forces. Had Reagan used the 1984 campaign to create a mandate for the policies he is now advocating, his agenda would be irresistible in Congress.

Instead, he took the easy way to a landslide, and now Congress feels free to go its own way -- up to the point that it threatens a personal confrontation with the president, whose power still awes them.

Not only did Reagan's 1984 victory lack the elements of policy mandate he embedded in his 1980 election, but it also lacked the coattails that had been there four years before. In 1980 Reagan's strongly issue-oriented campaign carried Republicans to a 33-seat gain in the House and a 12-seat pickup in the Senate. In 1984, though his personal victory was bigger, the Republicans lost two seats in the Senate and came up 10 seats short of their 1984 total in the House.

As Alan Ehrenhalt, the savvy political editor of Congressional Quarterly, has observed, Reagan probably lost the contra vote in the House back in 1984. If there had been 10 more Republicans and 10 fewer Democrats elected with Reagan, the 12-vote margin of defeat in the House on aid to the contras probably would have been reversed.

The passage of time only increases the consequences of Reagan's fateful choice of tactics in 1984. He is attempting to govern without a mandate, and a Congress conspicuously without courage is nonetheless challenging him effectively.

He cannot go back and rerun the last campaign. But he can acknowledge the reality of his own altered situation, and seek accommodations with members of Congress needed to achieve majorities and avoid the outright defeats that have marred his spring training season.