Washington — In a letter to President Ferdinand Marcos of the Philippines, President Reagan has stressed the American commitment to free and open elections. According to State Department officials, the American leader told President Marcos he remains convinced that continued movement toward fully functioning democratic institutions appropriate to the Philippines is the key to rebuilding economic and political confidence in that nation.
In the letter, which was delivered to Marcos on March 29, Mr. Reagan was also reported to have emphasized what he described as a deep commitment to representative government and a democratic electoral process which all Americans share. Reagan noted he had been kept well informed of ''steps being taken in the Philippines to encourage free and open legislative elections'' next month.
The American President was also reported to have assured Marcos that the United States considers the Philippines' elections to be strictly a Philippine matter, consistent with the worldwide US stance in such matters.
It was the latter point that was stressed in the Philippine press and in a Manila radio broadcast two weeks ago. Officials in Washington, considering this to be a case of selective ''leaking'' on the part of the Philippine government to emphasize one point, decided to set the record straight by giving a fuller account of the Reagan-Marcos letter, with its emphasis on the need for free and open elections.
The fuller account of the Reagan letter makes it clear that while the May 14 parliamentary elections are a Philippine matter, the Reagan adminstration is deeply interested in the outcome and has not yet come to a judgment as to whether the elections are likely to be fair.
The administration's interest in the Philippine elections derives from America's long and intense relationship with that Southeast Asian nation. The adminstration wants to maintain access to the Subic Naval Base and Clark Air Force Base in the Philippines, the two largest bases of their kind outside the United States. US officials describe those huge bases as the southern anchor in a defense chain stretching from Japan and South Korea through Okinawa to the Philippines. The bases became more important to the US after Soviet ships and planes began using former American bases in Vietnam on a regular basis.
Trying to influence President Marcos and the turbulent Philippines is a delicate matter. The administration does not want to use its influence in a manner that would weaken Marcos to the benefit of leftist-led Philippine insurgents. But administration officials are convinced that free and open elections would contribute to Philippine stability, whereas unfair elections might contribute to further instability.
The adminstration has distanced itself somewhat from President Marcos since the honeymoon period of mid-1981, when Vice-President George Bush, visiting Manila, declared: ''We love your adherence to democratic principles and practices.'' More recently, the administration has urged the Marcos government to pursue a vigorous, impartial investigation of last summer's assassination of opposition leader Benigno Aquino Jr. President Reagan postponed a planned trip to the Philippines last year.
In Manila, US ambassador to the Philippines Michael Armacost has spoken out on the investigation and the American interest in a process of ''dialogue and reform and freely elected government'' aimed at restoring confidence in the future and averting a ''polarization'' of Philippine political forces.
When it comes to the legislative elections in May, State Department officials see a mixed picture. In his letter, President Reagan spoke of steps being taken to encourage free and open elections. Officials explained the President was referring to a number of measures, including:
* An election law passed in February which incorporates voting safeguards.
* Provincial-based rather than regional-based elections.
* A new voter registration.
* The accepted role of the National Movement for Free Elections, a citizens' group monitoring the elections.
But State Department officials have also noted some troubling developments. One was the Philippine government's decision not to print all election ballots in one place. Printing of ballots at several locations could make it more difficult to keep track of the ballots and increase the possibility of ballot-box stuffing. The Philippine electoral commission appears to be heavily stacked in favor of Marcos supporters. Voter registration has been marred by allegations that ''flying registrants'' registered in more than one location.