Two Prince George's County residents announced last week that they plan to run for the job of county sheriff. One will challenge Sheriff James Aluisi in the Democratic primary; the other, a Republican, may be unopposed in the primary.

Reginald N. Riley, a 13-year veteran of the Prince George's Police Department, filed for the Democratic primary against Aluisi, a two-term incumbent. Riley, 35, announced his candidacy last week on the steps of the county courthouse to a gathering of 30 people, including a handful of deputy sheriffs.

Riley emphasized his law enforcement background, experience that he said would bring "professionalism" to the sheriff's department. "The sheriff's use of untrained volunteers has, to some extent, eroded the public perception of the professionalism of the department," he said.

Riley is a founder and president of the Black Police Officers Association of Prince George's County. A graduate of Fairmont Heights High School, he earned a bachelor's degree in law enforcement and criminology at the University of Maryland.

The only Republican to file for the office of sheriff is John Eugene Sellner, a retired county police officer who is seeking public office for the eighth time after seven unsuccessful campaigns.

Sellner, 56, of Fort Washington, is a former Democrat who now calls himself a "Lincoln Republican," which he said means "extremely conservative." Sellner said that the office of sheriff has not been attractive until now, with the salary increasing to $50,000 a year from $39,900.

Sellner said Aluisi's appearance in a National Rifle Association advertisement, his use of volunteer deputies -- some of whom are untrained -- and his periodic mass arrests of persons delinquent in child-support payments make him vulnerable in the coming primary.

Aluisi, 40, is expected to seek a third term as sheriff, but has not announced yet. Long to Leave State Senate

State Sen. Joseph Long, an Eastern Shore lawmaker and 24-year veteran of the legislature whose ever-present smile made him a friend of both conservatives and liberals, announced last week that he will not seek reelection.

In an emotional farewell to his colleagues, Long, 64, said it would be very difficult for him not to go on the campaign trail this fall.

"I've enjoyed my years in Annapolis," Long said at a rostrum at the front of the chamber, "and I'm going to miss it. The voters in my district have been good to me to allow me to represent them through all these years," he said.

The Democrat's district now includes Wicomico, Worcester and Somerset counties.

Long, an electrician, served on the Salisbury City Council four years before being elected in 1962 to the House of Delegates, where he served 16 years before being elected to the Senate.