Peter A. Bradford, a member of the Nuclear Regulatory Commission and a mainstay of its liberal faction, yeaterday announced that he will resign from the NRC this winter to become a public advocate in his home state of Maine.

Bradford's five-year term expires next June, and his chances for reappointment by President Reagan were regarded as nonexistent.His replacement will be Reagan's third appointment to the five-member NRC board.

The Senate is considering the second nomination, Thomas Roberts, 44, Vice President Bush's former presidential campaign treasurer and former head of the Southern Boiler and Tank Works in Memphis. Roberts is expected to be sworn in next week.

The first, Chairman Nunzio Palladino, took office July 1.

Bradford, 39, said he would become a member of the cabinet of Maine Gov. Joseph E. Brennan, representing the public interest in regulatory issues before the state Public Utilities Commission, where he served from 1971 to 1977, and in related court cases. Bradford will also serve on the state energy office advisory board.

He will take a cut to $37,000 from his current NRC salary of $55,000.

Nuclear power critics were unhappy over the news. "He always seemed to be the most responsive of the commissioners and has always shown a real concern about nuclear safety," said Anna Gyorgy, the new director of Ralph Nader's Critical Mass Energy Project. "We're sorry to see him go."

Rep. Edward J. Markey (D-Mass.), one of Congress' more outspoken nuclear critics, praised Bradford as "strong, independent and honest," and said his departure "will leave the balance on the NRC skewed against the American public and toward the interests of the nuclear power industry."

Scott Peters of the Atomic Industrial Forum, the industry trade association, said Bradford "has grave doubts about nuclear power and has been supercautious," occasionally voting "to obstruct nuclear power when there was no need to do so." His departure, Peters said, would "perhaps move things along" by allowing Reagan to appoint "someone who feels as he does."