Former Massachusetts Gov. Michael S. Dukakis, a liberal, is telling supporters that he will be making a comeback in 1982, Monitor correspondent George B. Merry reports. Mr. Dukakis, now a Harvard University professor, was unseated in the 1978 Democratic primary by conservative Edward J. King. He is the first candidate to declare for the office. At least one other Democrat, Lt. Gov. Thomas P. O'Neill III, is expected to vie with Mr. Dukakis to oust Governor King.