A Prince George's County state senator who had hoped to win a key gubernatorial appointment has been lobbying hard this week in an attempt to persuade the Maryland Senate's Executive Nominations Committee to reject Gov. Harry Hughes' choice for the job.

Committee members said Sen. B.W. Mike Donovan, a Democrat from District Heights who had hoped for a seat on the state's Public Service Commission, has been urging them to vote against Linda Thomas, a Western Maryland economist whom Hughes has nominated to fill the vacancy. Donovan had planned to resign from the Senate if he won the job.

The nominations committee is scheduled to meet Thursday to vote on Hughes' recommendation. In the meantime, Donovan is systematically working his colleagues.

"Just like Santa Claus, he's making a list and checking it twice," committee member Sen. Howard R. Denis (R-Montgomery) said of the lobbying tactics.

Several colleagues of Donovan, a real estate broker who served in the House of Delegates for 12 years before being elected to the Senate in 1979, said Donovan has indicated privately that he would like to step aside this year if another job could be found for him.

But Hughes overlooked Donovan and Edward Mason, a former state senator from Western Maryland who also was interested in the commission post, to choose Thomas, who worked as a legislative fiscal analyst for 10 years.

Donovan is "trying to kill Thomas' appointment because he feels he should have it," said Sen. Clarence M. Mitchell III (D-Baltimore), chairman of the Executive Nominations Committee.

Donovan declined to be interviewed for this article, but several senators, some of whom did not want their names attached to their comments, said that Donovan wants the job in order to avoid a hard political fight to retain his seat this fall.

Sen. Thomas V. Mike Miller Jr., a Democratic member of the nominations committee and the powerful head of the Prince George's Senate delegation, said that the governor's appointment of Thomas troubled him, and he accused the governor's office of trying "to muck up politics in Prince George's County."

Thomas appeared before the committee Tuesday with the full backing of Western Maryland senators, who said that the PSC seat traditionally has been reserved for an appointee from their part of the state. Under questioning, Thomas said she has lived in Frostburg, Md., for a year and a half but has been a registered voter in Allegany County only since March 19. She is executive director of an educational agency in Cumberland, Md.

"Unfortunately, Linda Thomas is being caught in the middle of Prince George's County's battle with the governor," said Sen. John N. Bambacus (R-Allegany).

Del. Albert R. Wynn (D-Prince George's) has announced his plans to run for Donovan's Senate seat but has received little support from the county Democratic organization, which Miller heads. If Donovan should retire before the end of his term, the senators would decide among themselves who his successor would be. Wynn would then have to run against an incumbent in September.