PHILIPPINES PRESIDENT Ferdinand Marcos, whose misrule is crushing his country, has finally done something encouraging. By calling a "snap" presidential election for Jan. 17, he has acknowledged the broad contention of his many critics, domestic and foreign, that the mandate he currently enjoys is inadequate and that the situation in the Philippines requires action. His offer opens up a vista that did not exist as long as he insisted he would serve until his term expires in 1987.

Most informed observers concluded some time ago that it was foolish to expect real reform from the authoritarian Marcos, an expert in manipulating the political game to his and his corrupt cronies' ends. The question, which had not been answered satisfactorily, was whether the United States might somehow undo Mr. Marcos' effort to board himself in behind his country's democratic forms and its usefulness to American strategic interests. With Mr. Marcos now committed to putting his power at risk in an early election, a new set of calculations enters in.

Mr. Marcos knows perfectly well, or should, how suspect are elections in which, as former foreign minister Raul Manglapus recently warned, "the dictator himself is to be a candidate, (retaining) all his absolute power and his control of the army, the Commission on Elections, the secret police, all national media, and all significant public and private funds. . . ." It follows that Mr. Marcos, to make sure his offer of elections serves his nation's interest, must outline promptly the procedure for fairness he has in mind. The traditional way for dictators to let go -- even those who, like Ferdinand Marcos, exploit the forms of democracy -- is to let go: to step down and yield the field to national forces, including the armed forces and the political parties. How else does Mr. Marcos expect to have new elections respected?

Cynics suggest, of course, that President Marcos has called early elections merely to embarrass his political opposition, which he has done his considerable best to keep weak and divided, and to still what he calls "childish claims" -- many emanating from the best American sources -- that he has lost touch with the people. If that is true, then the United States' job is cut out for it: to persuade Mr. Marcos to follow up his promise of elections with delivery. President Reagan is increasingly being drawn into the effort to save the Philippines from Ferdinand Marcos. He can expect no better opportunity for a long time.