On the eve of the Senate's first showdown vote on the fiscal 1986 budget, White House officials said yesterday that they fear setbacks on defense spending and Social Security while President Reagan is at the seven-nation economic summit in Bonn this week.

A senior White House official predicted that Reagan will win a preliminary vote scheduled today on the overall deficit-reduction package but could lose on such other, specific spending votes as those involving defense.

The official, interviewed on the condition he not be identified, seemed to be trying to lower expectations for success on the budget while Reagan is in Europe. The president is to return May 10.

U.S. budget and trade deficits are expected to be a major issue of discussion in Bonn, and Treasury Secretary James A. Baker III said yesterday that Reagan, if confronted there with criticism, will respond by saying that he will continue pushing Congress for spending cuts.

Meanwhile, Senate Republican leaders, after stalling last week to avoid defeat in a vote they had hoped to call then, prepared to push for a preliminary vote on the $52 billion deficit-reduction package in hopes of nailing down a victory through concessions to wavering GOP senators.

But, as of late yesterday, Majority Leader Robert J. Dole (R-Kan.) said he was not certain that he has enough votes to clear the first procedural hurdle and thereby claim a symbolic victory.

The White House official said the timing of the budget votes, while Reagan is in Europe, is "regrettable" and predicted "a whole series of stories" on specific amendments while the president is in West Germany this week.

While predicting victory on the first vote, he said the White House risks losing on items such as defense spending, where Reagan and the Senate GOP leadership had agreed on 3 percent growth after inflation.

The Senate, under pressure to save domestic programs, "may solve this by hitting the hell out of us on defense," the official said.

He added that White House officials, who conducted a legislative strategy meeting yesterday, are "very concerned" that the Senate may also reject the GOP plan to cut Social Security cost-of-living adjustments roughly in half.

The official said that Democrats are opposing the White House-Senate GOP compromise on the first vote but that some could be expected to join Republicans on specific amendments.

Among concessions yesterday was an agreement to save the Rural Electrification Administration at a reduced level of spending instead of phasing it out over eight years. Sources said the move nails down the support of Sen. James Abdnor (R-S.D.) on the preliminary vote.

In a previously reported concession, Senate leaders agreed to restore $220 million for programs for the handicapped in an attempt to win over Sen. Lowell P. Weicker Jr. (R-Conn.). Seeking further spending restorations, Weicker remains uncommitted on the first vote.

In addition, Sen. Charles E. Grassley (R-Iowa) said he was persuaded to go along with the leadership on the first vote when it guaranteed him a chance to offer a substitute proposal to freeze spending at current levels at the end of the budget deliberations.

He said he also was assured an opportunity to offer an amendment to raise defense spending only to adjust for inflation.

Dole indicated that he is attempting to win over other dissident Republicans by offering to introduce their pet amendments, even though he opposed them.

This could be critical because of the high stakes in partisan jockeying over restoration of the full cost-of-living increase for Social Security benefits, which Sens. Paula Hawkins (R-Fla.) and Alfonse M. D'Amato (R-N.Y.) have been seeking to champion.

Under Dole's offer, Hawkins and D'Amato would beat Democrats to the punch in "saving" the increase. Sen. Daniel Patrick Moynihan (D-N.Y.) has said he will try to take the lead in scuttling the White House-blessed proposal to halve the increases over the next three years.

"Why not?" Dole asked in discussing the strategy with reporters. "Why let Moynihan do it when he's never going to vote for it the package anyway?"

Among other issues that Dole said he may throw out for a vote on the Senate floor to satisfy Republican waverers are a defense-spending cutback and a corporate minimum tax.

Dole said late yesterday that he does not know whether his strategy will work.

It was also unclear whether the Democrats, who had been pressing for a vote so long as Dole appeared to be facing defeat, would allow a vote now that Dole may have picked up enough votes to win and to give Republicans credit for any subsequent changes in the plan.

Minority Leader Robert C. Byrd (D-W.Va.) indicated that he still wants a voting arrangement under which Democrats would get the first crack at sponsoring the Social Security amendment.

In an address yesterday to the annual meeting of the U.S. Chamber of Commerce, Reagan acknowledged that the budget compromise will "run into stiff opposition." He appealed for support on it even though it would reduce business subsidies.

"In a fundamental sense, what is at stake is nothing less than our democracy itself," Reagan said.

He asked the Chamber to "hold a few feet to the fire for me" and "help to knock some sense into the spenders" in Congress.

Although his senior aides said as recently as last week that the U.S. economy is not strong, the president declared yesterday, "The economy is healthy and growing. When we put our plan into effect, it is certain to grow still stronger."