Prince George's County Del. Thomas J. Mooney will announce today that he is switching parties and becoming a Republican, making him the only GOP member of the county's legislative delegation.

Mooney, a 41-year-old legislator from Takoma Park, said he will announce the change in party affiliation at a morning press conference in Laurel.

Last year, Mooney headed a Democrats for Reagan group in Maryland, and there had been persistent reports since that he would abandon the Democratic Party. He is serving his second term in the House of Delegates.

Mooney said last night that he is changing parties because, "intellectually, I felt I was in a dry-rot cycle, that I was stagnant."

Mooney denied rumors circulating through county Democratic circles that he would eventually resign his seat to take a promised job in the Reagan administration.

"I haven't been offered a job, and I am still a member of the House of Delegates," he said. "There is no quid pro quo."

Mooney becomes the second Democratic officeholder in Maryland to switch to the GOP in recent months. Earlier, Howard County Executive J. Hugh Nichols became a Republican. He is expected to seek the gubernatorial nomination as a member of that party.