The big difference between early April, when my vacation began, and early May is the wobble in President Reagan's commanding political position. In the eyes of Washington politicians of both parties, his leadership has clearly been shaken. What remains to be determined is whether it is a temporary setback or the start of a serious second-term slide.

In the few weeks I was wandering happily through museums, gardens and caf,es in Spain, the president suffered a series of unexpected and uncharacteristic setbacks: the self-inflicted wound of the Bitburg cemetery controversy; the rebuff by Congress to his Nicaraguan policy; the failure of his budget speech to mobilize any visible public response.

None of these in itself seems serious or long-lasting enough to warrant the sweeping and negative reappraisals of his leadership position I have heard from politicians here. Bitburg was a blunder, but not a policy catastrophe; it is not likely to be a continuing wound. As for Nicaragua and the budget, Reagan is still very much in the game, as subsequent events have shown.

The president came into this bad patch on as much of a political high as I can remember seeing in the last 25 years. His reelection victory confirmed his control over the policy agenda of the nation, and the way seemed clear for him to push it vigorously in his second term. The political opposition was demoralized. His own party was operating with increased self-confidence, based on evidence of its progress toward majority status in the nation.

He was, in short, comfortably cushioned to withstand a few short-term reversals. Why, then, the spreading sense that these past few weeks could turn out to have been a turning point toward long-term decline?

There are three reasons -- none of them as yet certain, but all of them substantial.

The first is that the common thread in the three recent failures is a misjudgment of the situation by Reagan and the White House staff. In all three cases -- Bitburg, Nicaragua and the budget package -- the decision-making was dubious, the president's ability to defend his decision was overestimated, and the extent of the opposition was undervalued.

This president has profited greatly from the belief that it is not safe to oppose him. On all three of these issues, people did and suffered no bad consequences.

That is a lesson that carries dangerous implications for Reagan, even more dangerous than the widely expressed judgment -- from congressional Republicans, from top business lobbyists, as well as from Democrats -- that the new White House staff dominated by Donald Regan and Patrick Buchanan is not the equal, in political finesse, of the old one led by James Baker.

That conclusion may or may not be warranted, but it has been made by politicians here. Such judgments tend to be self-fulfilling.

The second factor is outside the president's control, but it also poses a long-term threat to his dominating position. That is the emergence of Mikhail Gorbachev as a personality on the world stage with some of Reagan's presence and persuasiveness.

One reason the president has loomed so large is that the other players in domestic and world politics appeared to be midgets. No one in Washington is remotely on his level as a public personality. The only foreign figures who have that special dimension -- Pope John Paul II, Margaret Thatcher and Deng Xiaoping -- have their own very diverse but very compelling reasons for helping make the president look good.

In Gorbachev, for the first time, Reagan faces an adversary who may have some of the president's own skills as a manipulator of public opinion. If Gorbachev employs those talents, not to forge an arms control deal with Reagan and revive d,etente but to try to undermine the Western alliance and isolate the United States, he could pose a real threat to Reagan's hopes for a major foreign policy success to crown his second term.

The final new element is even more worrisome: the emergence of a series of warning signs of a fairly drastic slowdown, or even a decline, in the American economy. All Reagan's domestic goals -- deficit reduction, tax reform, a further shift of responsibility away from the federal government -- depend on continued healthy economic growth.

The notion that he is a "Teflon president," immune from blame when anything goes wrong, has never been true. Like every other president, he is held directly accountable for the health of the economy. When the economy was in the pits at the end of 1982, Ronald Reagan, for all his charm and eloquence, had a dangerously low approval rating.

If this president and this White House staff find themselves confronting both a recession and a resourceful, antagonistic Gorbachev, then the wobble of the past few weeks could foreshadow a collapse of his political strength.

That is what many of the Washington insiders seem to be anticipating. But they have misjudged, and grossly underestimated, Ronald Reagan's resiliency before.