Reagan's gradual slide in popularity helped little by European trip. The President's public support has dipped since January, when his approval rating in the polls was nearly 70 percent.

Bitburg hurt. But White House advisers predict the President's troubled European trip could still give Mr. Reagan a small boost in the polls, even if his standing with some Jewish groups and veterans has eroded. Richard Wirthlin, the President's pollster, says, ``The White House calls are going 3 to 1 in favor of his going to Bitburg.'' But he adds, ``This was clearly not a classic European presidential trip. And the consequences are still very much to be determined.''

Washington insiders are watching the aftermath of the European trip with extra interest because a serious loss of Mr. Reagan's standing with the public could undercut his embattled programs on Capitol Hill.

Defense spending, aid to Nicaraguan rebels, and a host of other Reagan policies could be hurt if Congress sensed that Mr. Reagan were losing his enthusiastic support from the public.

G. Donald Ferree, associate director of the Roper Center, an archives for poll data, observes that Mr. Reagan ``remains very popular.'' Mr. Ferree says this is true, even though the ``euphoria after the election has begun to dissipate.''

One factor helping the President right now, says Ferree, is that there is no widespread public acceptance of any alternative policies to those of Mr. Reagan. The opposition has no programs around which it can rally.

Even so, several developments in recent weeks have touched off alarm bells in the White House.

Dr. Wirthlin, in a breakfast meeting with reporters yesterday, said polls taken for Mr. Reagan have shown the President's popularity slipping gradually since January, when his approval rating with the public was nearly 70 percent. In the latest poll, April 27, it had declined to 61 percent. That's still high, but it bears watching, he said.

Then there was Bitburg. Norman Ornstein, a scholar at American Enterprise Institute, observes that Bitburg raised questions about the political savvy of Mr. Reagan's second-term team, headed by chief of staff Donald Regan. Another mistake like this one, he said, could begin to make Mr. Reagan ``look like Jimmy Carter, who had the image of a bumbler. Is Bitburg the exception, or the rule, in Reagan's second term?'' he asks.

Distracted by Bitburg, the Reagan team has suffered a string of defeats on Capitol Hill.

In the Senate, his budget has been battered. His 3 percent rise in defense spending was cut to zero; his reductions in social security were restored.

In the House, a $14 million aid package to Nicaraguan rebels was defeated. Although Democrats are now backtracking on that issue, the Nicaraguan vote made the President's strategists appear inept.

Meanwhile, there has been an important shift of public opinion taking place on the issue of the budget deficit. The deficit, a relatively quiescent concern throughout 1984 despite Walter Mondale's efforts to publicize it, began to simmer early this year, and now is bubbling actively.

As recently as January 1985, Dr. Wirthlin's polls had showed that only 1 percent of all American voters considered the federal budget deficit to be the country's most important problem. That has dramatically changed. Today in some states, such as those in the Farm Belt, it is now rated the No. 1 concern, Wirthlin says.

At the moment, however, it is the Bitburg aftermath that most concerns the White House.

Wirthlin notes that an ``emotional tearing'' took place among some voters because of Bitburg. It will take time to see what the grass-roots effects of this will be, says Wirthlin.

Dr. Ornstein suggests there will certainly be short-term damage, but feels the long-term impact probably will be limited, even in the Jewish community.

Although most Jewish voters supported Democrat Mondale in 1984, says Ornstein, the Jewish community has been sympathetic to Mr. Reagan. Until now, most of them have not been ``out on the barricades against him,'' and some have supported him, he observes. ``The fact that Reagan got only 35 percent of the Jewish vote was less significant than the fact that [Jewish] opinion leaders were positive, or at least not strongly negative about Reagan.''

Ornstein sees no long-term damage to relations with Israel. Denunciations of Reagan by Israeli leaders this week were just ``domestic politics'' over there, Ornstein suggests.