Without actually changing policy, the Reagan administration is changing its description of where it stands on the possible use of troops in El Salvador in an effort to preempt what it sees as rising congressional opposition.

"We are trying to outflank the Democrats," one well-placed White House official said in California yesterday.

President Reagan, who is in California, previously has said only that he has "no plans" to send U.S. combat troops "anyplace in the world," and has been careful not to go beyond that formulation.

Now aides are saying privately that Reagan has virtually ruled out such use of troops; that he doubts the Pentagon could win a short-term victory in El Salvador except with a large commitment that would involve diversion of U.S. forces from other vital regions, and that in any case he doubts such a victory would produce social stability in the region over the long run.

A number of aides have made such remarks to a number of reporters in recent days. The remarks come amid rising political uneasiness over Reagan's intentions in Latin America.

Just yesterday, Senate Minority Leader Robert C. Byrd (D-W.Va.) moved to meet that uneasiness, saying he will introduce on Monday an amendment to the War Powers Act that would require prior congressional concurrence before U.S. combat troops could be sent to El Salvador.

White House deputy press secretary Larry Speakes said yesterday that such an amendment isn't necessary. While refusing to rule out military action as a theoretical option, Speakes said that the administration believes that the U.S.-backed El Salvadoran government can prevail against rebel troops with U.S. military assistance and without the sending of any American combat forces.

"There is no plan to send combat troops to El Salvador or anywhere else," Speakes said.

Byrd said he was worried about a creeping escalation of administration rhetoric on El Salvador and the possible future use of troops. "I don't want to see escalating rhetoric followed by esclating involvement . . .such as we had in Vietnam," he said.

The War Powers Act, passed in 1973, allows the president to commit troops to combat on his own but requires him to seek congressional approval of such action within 60 days.

Earlier, Speakes told reporters that the notion of sending U.S. troops to El Salvador is "an exaggerated idea" and implied that fears of such action are an invention of the administration's political opponents.

Another official said bluntly that the administration is engaged in "political warfare" with opponents of its El Salvadoran policy and added that there have never been any plans to send U.S. forces into El Salvador beyond the few advisers who are helping to train El Salvadoran soldiers.

Administration officials acknowledged, however, that they face a difficult political problem on the troops issue because President Reagan has consistently refused--as he did at his last press conference--to flatly rule out the use of American combat forces in the region.

It is Reagan's view, and the view of Secretary of State Alexander M. Haig Jr. and national security adviser William P. Clark, that such an inclusive statement would give the rebels a firm assurance that might make it more difficult for the El Salvadoran government to prevail.

What Reagan and his top officials want is, in effect, the best of both worlds. In El Salvador, they would like U.S. military intervention to be seen as a last-ditch option if the rebels appeared to be winning. On Capitol Hill, they would like congressional leaders of both parties to accept the administration's premise that no intervention is contemplated and that any analogies between El Salvador and Vietnam are misplaced.

While Speakes was talking to reporters in the White House press center in Santa Barbara, Calif., 18 miles from the president's mountaintop retreat, about 200 persons opposed to any U.S. intervention in El Salvador demonstrated across the street from the press center.