The attitude of United States officials here toward the government of Ferdinand Marcos seems to have changed sharply in the last two months. Immediately after the assassination of opposition leader Benigno Aquino at the Manila airport on Aug. 21, the US attitude seemed to be one of cautious support for an embarrassed ally. Diplomats here appeared disposed to accept the government's claim that the killing was the work of the communist underground.
Today, the sympathy and complacency have apparently disappeared. US officials are reportedly gloomy about the country's economic and political prospects.
They are also critical of the government's handling of the crisis, deeply skeptical of the official version of the murder, and concerned at the role the military may play when Mr. Marcos leaves the scene.
And they are described as lobbying the President, so far without success, to make concessions that will alleviate the present crisis.
The main cause of this change appears to have been the response of the Filipino middle class to the assassination. The massive, seemingly almost universal, sense of outrage that has brought millions of people out into the streets of Manila appears to have shaken US diplomats here.
''These people - the business class, the middle class, and the professionals - are the future of the economy,'' a high-level US official says.
If the government is unable to restore its credibility in the eyes of these people, the official says, it would be unable to obtain the vitally needed ''new money'' - investment to revive the economy rather than simply pay the country's debts.
The loss of middle-class confidence, the senior US official reportedly feels, has precipitated the current economic crisis. The Philippines' foreign reserves are dwindling fast. They are thought to stand at around $300 million, enough to pay for two weeks' worth of essential imports. The country's balance-of-payments deficit stands at an all-time high - $2 billion - and foreign bankers are not rushing to lend more money.
The government has to restore confidence fast. The present economic and political crisis has what one source close to the US Embassy calls a ''short time fuse.'' It has to be resolved by the end of the 90-day debt moratorium declared by 10 of Manila's largest foreign creditors on Oct. 14.
A reliable source here says that the US has been urging President Marcos to take a number of measures to restore middle-class confidence. These include:
* Demonstrably impartial and exhaustive investigation into the murder of Mr. Aquino.
* Changes in the electoral rules here, which are seen as favoring the ruling party.
* A clear formula for succession, should President Marcos die or be incapacitated in office.
These measures have not, however, been taken, although recent changes announced in the electoral rules have ''heartened'' the business community, says the source close to the US Embassy here.
On the other hand, US diplomats are apparently suspending disbelief on the new commission of inquiry, which held its first session Thursday, Nov. 3.
''At least it can't be discounted at the outset like the first commission,'' says the source.
But Marcos's statement on the succession earlier this week has reportedly worried rather than reassured American officials. The main point of concern is the important role assigned in the succession formula to the secretary-general of the ruling Kilusang Bagong Lipunan (Movement for a New Society, or KBL).
The present KBL secretary-general is Deputy Premier Jose Rono. But, as a source close to the embassy notes, Mrs. Marcos has been telling ''everyone'' that she will soon become secretary-general.
The possibility that Mrs. Marcos will play a major role in deciding the succession - and perhaps even succeed her husband - seems likely to further diminish rather than restore middle-class confidence in the regime.
US officials here are also thought to share the general skepticism with which the official murder story is viewed.
''Circumstantial evidence has already cast a fair amount of shadow over the official version,'' says one well-informed source.
The same person was also scornful of the official government probe led by Gen. Prospero Olivas, Metropolitan Manila police commander and an intimate of those in the presidential palace.
''A cadet in a police academy,'' says one high-level US official, ''could have found out more in one hour than Olivas did in weeks.''
One reason for the President's unwillingness to act on the embassy's proposal may be the opposition of a small group of palace advisers whose considerable influence over the President is a cause of concern in political and diplomatic circles.
The core of the inner group is usually believed to include Mrs. Marcos, Gen. Fabian Ver (a relative of the President whose titles include armed forces chief of staff), and Benjamin (Kokoy) Romualdez, Mrs. Marcos's brother. Two major businessmen - Roberto Benedicto and Eduardo Cojuangco - are probably included in the group as well.
''There are people over there (at Malacanang Palace) who want to play political hardball,'' says a source close to the embassy. He declined to name the hardball enthusiasts, but they probably include most or all of the inner group.
The hardliners are said to argue consistently against any political concessions, which they reportedly maintain would be viewed as signs of weakness or as an admission of guilt.
This, US officials here feel, is one of the key problems in the present political crisis: ''The President gets overtaken by events. He holds out, then makes gestures too late,'' thereby losing the credit he would have gained.
Like most other political observers in Manila, US diplomats here are certain that the President is often in poor health, but they are unsure of the gravity of his illness.
Their prognosis for the country, however, is said to be bleak. They feel that Mr. Marcos will get through the present crisis.
''That is,'' one source explains, ''he won't resign.''
But the country is unlikely to resume a healthy economic growth rate for several years. And according to one US official, ''The restoration of the government's legitimacy would require some steps that haven't yet been made.''
And another factor seems to worry US diplomats as they look to the future: the role of the military.
In addition to its considerable political clout, the armed forces have also branched out into what one source heredelicately calls ''economic enterprises'' - car theft, smuggling, and black marketeering.
These undertakings will probably be more closely monitored in the present atmosphere of economic austerity.
Moreover, the military may not prove as faithful to Mr. Marcos's successor as they have been to the President.
''Whoever inherits this situation will have a lot of trouble with the military,'' says a US observer.