A former Washington lobbyist who was fired from his job with an energy company after he wrote a letter to Interior Secretary James G. Watt yesterday filed a $6 million lawsuit in U.S. District Court here against Watt, one of his aides, his former company and its president.

The lobbyist, Timothy L. Donohoe, who worked for the Dallas-based Enserch Corp., had asked Watt in the letter to explain what he meant by reported statements that he divided the political world between "liberals and Americans." A Watt aide, Stanley W. Hulett, sent a copy of Donohoe's letter to Enserch President William C. McCord, saying that Watt was "frankly surprised" at Donohoe's letter. McCord then fired Donohoe.

In his lawsuit, Donohoe contends that Watt authorized Hulett to get him fired and that Watt and Hulett intended to punish Donohoe for exercising his right to free speech. Donohoe also claims that his company, which has lease applications pending before Interior, fired him "to curry favor" with officials there.

A spokesman for Watt has said that the secretary never saw the letter Hulett sent to Enserch president McCord. A spokesman for Enserch has said Donohoe was fired for the "bad judgment" he used in identifying himself in the letter to Watt as an energy company lobbyist.