The recent switch in allegiance of a Philippine armed forces colonel to an opposition group in Washington appears to have struck a raw nerve in the government of President Ferdinand Marcos.

The action last week by Col. Alexander Bacalla, the assistant deputy chief of staff for civil-military operations at general headquarters, represents the strongest expression to date of tensions and morale problems plaguing the estimated 150,000-member armed forces, according to Philippine military sources and political analysts.

In what was interpreted as a sign of this concern, the Air Force has forbidden its officers to take foreign vacations, the official Philippine News Agency reported Thursday. The order said that leaves abroad would be granted only for medical treatment or study grants, according to the agency.

Announcing what he called his defection at a meeting in Washington of the Movement for a Free Philippines headed by self-exiled Philippine opposition leader Raul Manglapus, Bacalla said he was joining the group as a "foot soldier" in the fight to "regain the basic freedoms our people have lost under the Marcos dictatorship." Copies of his statements were circulated here.

In Washington, Bacalla said he plans to ask for political asylum in the United States because "I place my life in jeopardy if I go back to the Philippines." The State Department had no comment on the case.

Immediately after Bacalla announced his defection, the Philippine government sought to discredit him, asserting that he "deserted" for personal rather than polical reasons. The military here issued a statement saying that he had deserted to avoid disciplinary action for abandoning his family and "cohabiting" with another woman, whom he had brought to the United States.

The controversy comes as Marcos faces U.S. pressure to reform the military by eliminating human rights abuses, weeding out corruption and increasing efficiency and morale. The U.S. aim is to improve the military's capability to combat a growing Communist insurgency, which Washington insists is more serious than Marcos is willing to admit.

Marcos has said that he intends to replace generals who have served beyond retirement age with younger officers to improve efficiency and morale. But one military source said Marcos' statements were at odds with recent actions, such as extending for six months the service of several older generals.

The military has become a source of increased sensitivity for the Marcos government lately because of reported uneasiness among reformist officers over the likelihood that Gen. Fabian Ver, a staunch Marcos loyalist and distant cousin, will be acquitted in the country's most celebrated murder trial and reinstated as armed forces chief of staff.

Ver was charged along with 24 other military men and one civilian with involvement in a plot to assassinate opposition leader Benigno Aquino in August 1983 and cover up the crime. A verdict in the case is expected soon, and Ver's acquittal is widely seen as a foregone conclusion since virtually all the evidence against him was thrown out during the trial.

Marcos has said repeatedly that he would reinstate Ver if he is acquitted despite the evident objections of Washington, which regards Ver as practically the embodiment of what is wrong with the armed forces.

"The reformists are bracing themselves for a tough time ahead with the return of Ver," said an independent observer with close military connections. He said the "heady atmosphere" that prevailed when younger officers who support changes in the military banded together earlier this year is now gone, replaced by foreboding that "there will be a certain amount of repression against reformist officers when Ver gets back."

In any event, he said, "the reformists have concluded they cannot reform the armed forces from within." Some have privately expressed dissatisfaction lately with the performance of acting chief of staff Lt. Gen. Fidel Ramos, a West Point graduate who has expressed support for the reformists but who has proved unable to implement bold changes.

Similar views were aired in Washington last week by Bacalla, 47, a graduate of the Philippine Military Academy with 27 years of military service. Bacalla said his latest position was number two man in the J7 department of the general staff responsible for propaganda and psychological warfare.

"Under the Marcos dictatorship, that uniform -- in the eyes of our people -- drips with blood, the blood of countless victims of military atrocities and the blood of Aquino," he told opponents of the Marcos government. Bacalla blamed Marcos as "the man most responsible for the deterioration of the armed forces," which he said had turned "into an instrument of personal rule."

The colonel said he had not defected earlier because he believed the military "could reform itself from within." However, he said, he realized this was "next to impossible" under the present leadership because Ver, while ostensibly on leave, "remains in effective control of the armed forces of the Philippines."

Bacalla said his views were shared by about 70 percent of the Philippine officer corps. He said he hoped his defection would "embolden them to press for meaningful reforms more forcefully and vigorously in an organized manner."

While no other such actions have been announced yet, reformist military sources here said Bacalla's complaints struck a responsive chord.

"There are a number of officers who are entertaining the same sentiments that this colonel publicly pronounced," said one officer, who did not want to be identified. "Loyalty to the constitution and country has been supplanted by loyalty to Marcos. The emphasis on loyalty rather than qualifications is one of the main grievances of the reform movement."

The officer said Marcos was "very concerned about the reform movement," fearing that "defections will inspire it."

In a statement issued here, the movement, formerly called "We Belong" and now known as the "Reform AFP Armed Forces of the Philippines Movement," said Bacalla "is a respected colleague who doubtless acted honestly according to the dictates of his conscience."