District 3, which covers the northeast part of Washington County and the western half of Frederick County, now is represented by Republican Sen. John W. Derr, the minority whip in the Senate.
No one else has filed to run for the seat, according to the state Board of Elections.
Derr, who has been in the Senate since 1983, said Tuesday he won't decide whether to seek re-election until after next year's legislative session, which ends in April.
"It's far too early," Derr said.
If Derr runs, Mooney said he will position himself as a conservative alternative to an incumbent he considers moderate-to-liberal. Mooney said he will campaign against Derr's opposition to an elected school board in Frederick County.
Mooney doesn't see his age as a deterrent to getting elected or serving in Annapolis.
"One of the advantages of youth is you go down there with a fighting spirit," he said.
A 1989 graduate of Frederick High School, Mooney has a degree from Dartmouth College. He has worked for U.S. Rep. Roscoe G. Bartlett, R-Md., and the House Republican Conference. For the past 2 1/2 years he has been working for CNP Action Inc., a conservative business policy organization.
State legislators in Maryland are paid $29,700 a year.
The deadline to file for the election is July 6, 1998. The primary is Sept. 15, 1998, and the general election is Nov. 3, 1998.