Reagan administration efforts to speak with a single voice on Central America and other touchy foreign policy questions have been frustrated by a growing struggle between White House communications director Patrick J. Buchanan and key aides who played important roles during the first Reagan term, according to well-informed officials.

"Pat is pushing the president into confrontation with Congress and the press, two key constituencies with which Reagan has always done pretty well," said one senior official.

Such veterans as deputy chief of staff Michael K. Deaver, national security affairs adviser Robert C. McFarlane and political adviser Edward J. Rollins take pride in President Reagan's first-term success in coaxing Congress into compromises and scoring points in public appearances.

Now, there is a new anxiety among longtime aides and Reagan advisers outside the White House about Buchanan, who was described by one of them as "far more interested in the success of the conservative cause than the welfare of the president."

Buchanan's combativeness has led to expressions of concern in the White House about the judgment of the man who hired him, chief of staff Donald T. Regan. An administration official said yesterday that Regan, although an effective manager, had "simply no idea of how Buchanan would try to mobilize the conservative constituencies to get his way."

Officials of all views agree that Buchanan is persistent, energetic and far more able at advancing his views than many of his rivals. Some attribute this to his experience in the embattled White House of President Richard M. Nixon, where Buchanan was a speechwriter and loyalist.

Buchanan's persistence was particularly evident in the past two weeks on the issue of providing aid to the Nicaraguan rebels. Buchanan's recommended strategy put him into direct conflict with McFarlane, who favored private consultations with members of Congress to find a consensus.

First, Buchanan pushed for a nationally televised speech on the issue last Sunday. When this approach was rejected because Senate Majority Leader Robert J. Dole (R-Kan.) and other White House aides wanted a speech on the budget instead, Buchanan kept pushing.

On Saturday, Reagan gave a radio speech on the Nicaraguan issue saying that failure to give aid to the rebels would be "a shameful surrender." On Sunday, Buchanan conferred with Central Intelligence Agency Director William J. Casey and other administration conservatives and with Sen. Jesse Helms (R-N.C.), then tried unsuccessfully to persuade Reagan to give a speech the following night describing aid to the rebels as essential to U.S. security.

Buchanan, a conservative columnist and commentator before he joined the White House staff in February, also has continued to clash with Deaver, a longtime friend of the president, over communications strategy.

An administration official said that Buchanan recently arranged for conservative religious broadcaster Pat Robertson to interview Reagan on his religious beliefs. Deaver and other officials, concerned that the interview might provoke new controversy during a difficult period in the presidency, sidetracked it and Robertson was sent to question budget director David A. Stockman about economic issues.

Nevertheless Buchanan, who started in February with control of the presidential speechwriting operation, has been successful in expanding his authority within the White House. He prevailed over Rollins and won control of the public liaison office, and he bested White House spokesman Larry Speakes in a contest over who would manage the offices responsible for dealing with out-of-town news organizations.

Buchanan's confrontational style increasingly has been evident in Reagan's speeches. He is credited with Reagan's phrase describing the Nicaraguan rebels as "the moral equivalent of the founding fathers" and with the president's characterization of World War II German soldiers and SS troops as "victims" of the Nazis "just as surely as the victims in concentration camps."

Some White House aides say Buchanan is concerned with rhetoric, not policy. Officials close to McFarlane tried to draw that distinction last week as the security adviser worked with sympathetic congressmen to devise a formula that would persuade a reluctant Congress to provide "humanitarian aid" for the Nicaraguan rebels.

Other officials say, however, that the distinction is artificial -- that rhetoric often is policy.

Reagan always has placed great emphasis on his public speeches. His phrases "evil empire" and "focus of evil" to describe the Soviet Union came to define superpower relations in the early stages of the first term. The softening of this rhetoric by Reagan's political strategists marked a turning point in the president's first-term dealings with the Soviets.

Several senior officials say they are concerned that Buchanan's combativeness will ruin the prospects of winning support on foreign policy issues from congressmen who have been susceptible to Reagan's charm and personal lobbying.

"It's pretty hard to ask Democrats to go along with you on Tuesday when you have painted them the party of surrender on Saturday," observed one official.

Buchanan has followed a policy of refusing to return phone calls from reporters, although officials say he has been helpful to a few conservative columnists.

"How can you be director of communications and not return press calls?" one veteran Republican strategist wondered. "It's got to be some kind of a joke."

There is widespread agreement at the White House that Buchanan isn't kidding about wanting to push the president to take the path of confrontation.