State Del. Thomas Mooney of Prince George's, a former Democrat who switched to the Republican Party this fall, says he is considering a run for the U.S. Senate seat being vacated by Republican Charles McC. Mathias Jr.

Mooney said he will file the required papers for his campaign committee this week and begin asking party leaders for their support.

"Philosophically, I guess I consider myself a compassionate conservative," said Mooney, 41, a former real estate agent and political consultant who was first elected to the General Assembly in 1978. "I'm not any longer a person of the '60s; I feel a lot of those programs have failed.

"But the Reagan administration has too often been less than compassionate. I think there are segments of society that have been overlooked in the rush to keep inflation down."

Since Mathias announced in September his plans to retire next year, the party has unsuccessfully sought a well-known candidate. Among the top choices was former U.N. ambassador Jeane Kirkpatrick, who declined, leading to fears that the Democrats would regain the seat.

Four relatively well-known Democrats are contenders for the seat: Reps. Michael Barnes of Montgomery County and Barbara Mikluski of Baltimore City, Baltimore County Executive Donald P. Hutchinson, and Gov. Harry Hughes, who has not formally announced that he will run.

Allan Levey, state GOP chairman, said his spirits have been raised in recent weeks by the appearance of a number of potential senatorial candidates, including former U.S. attorney George Beall, White House public liaison director Linda Chavez, developer and former ambassador Kingdon Gould, Vietnam Veterans Memorial founder Jan Scruggs and businessman E. Brooke Lee, a dark horse mayoral candidate in Washington in 1982. Only Lee has formally declared his candidacy.

Mooney, known in Prince George's as much for his whimsical campaign tactics as his maverick Democratic status -- he once delivered forget-me-not seeds along with his literature -- vowed to "walk the state of Maryland" to get support.