After 12 years out of office, former senator Eugene J. McCarthy announced formally today that he is seeking a political comeback in the Senate as a "reflective" veteran with expertise in economics, foreign and defense policy and the evils of the federal bureaucracy.

At a news conference in the state capitol, McCarthy, who will be 66 March 29, said he would ask for endorsement at the Democratic Farmer Labor Party state convention in Duluth June 5. But if he fails to get the endorsement he will run in the Sept. 14 primary, he said.

McCarthy, who served 22 years in Washington, 12 in the Senate, is the third declared Democratic candidate for the Senate seat held by Republican David F. Durenberger. The others are Mark Dayton, 34, a millionaire heir to a department store fortune, and Grant Merritt, 47, a lawyer who headed the Minnesota Pollution Control Agency between 1971 and 1975. Neither can claim front-runner status.

McCarthy, who endorsed Ronald Reagan for president, was harsher in his comments about the Carter administration than the Reagan administration. When a reporter said Democrats were calling the Reagan administration a disaster, McCarthy said: "I don't think it has been a disaster yet."

Asked if he would repudiate his Reagan endorsement now that he is running for office, McCarthy said it was so mild, limited and modest that there would be little difference between it and repudiation.

McCarthy won a place in history as a 1968 presidential candidate opposed to the Vietnam war, but he said today that he didn't think the Reagan administration's Central American policy could be compared with Vietnam "in a major way."

In his written statement announcing his candidacy (which was accompanied in the press packet with a poem by Cadock, an ancient Irish poet) McCarthy recited his experience in economic, foreign, defense and farm policy when he was in office. He told reporters that he could bring a "leveled-off and reflective" attitude, now that he was past age 60, to the burning issues of the day, such as nuclear arms control.

McCarthy, who has been occupying himself as a writer, lecturer and poet in Woodville, Va., described himself today as a Minnesotan.