Perhaps one of the most fiercely contested races in Maryland this year is the campaign for the Democratic state Senate nomination in Prince George's County's 24th District.

But it appears to have one unique feature: two incumbents.

Sen. Decatur W. Trotter has held the seat since 1983, so he's the incumbent. Right? Then why does the campaign sign at Martin Luther King Jr. Highway and Glenarden Parkway read: "Re-elect Tommie Broadwater, State Senate"?

Broadwater can argue that "re-elect" is technically accurate, because he was elected three times to the Senate in the district, which includes Glenarden, Seat Pleasant, Colmar Manor, Brentwood, Fairmount Heights and Capitol Heights.

But he left the seat in 1983, when he was convicted and imprisoned for food stamp fraud. Despite Broadwater's protestations, Trotter was appointed to fill the vacancy and was elected on his own in 1986.

Broadwater's explanation for the campaign poster: "I never was unelected." In fact, he said he hasn't heard complaints about the advertising "because it's true."

"Some people may have problems with it," he said. "If they do, that's their problem."

For his part, Trotter is acting unconcerned.

"I'm not worried about that. Maybe some people who have just moved into the district might get confused," Trotter said. "But the people who come out and usually vote, they know the difference between Tommie and myself." Poisoned Pens

In Montgomery County, the two Democratic Senate candidates in District 17 -- Sen. S. Frank Shore and Del. Mary Boergers -- have been conducting a war of open letters.

Shore, in a letter distributed by his campaign, first congratulated his challenger on her attendance record in the General Assembly, then questioned her acceptance of campaign contributions from political action committees.

"Whose business are you tending to? The people's business . . . or just business?" Shore wrote.

" . . . We both know that PACs and big businesses do not just give away money out of the goodness of their hearts. They expect a return on their investment," the letter said.

In a reply letter circulated by her campaign, Boergers wrote that Common Cause in 1987 listed her as receiving $11,990 from PACs and Shore getting $9,468.

"What's the difference?" Boergers asked. "You can't have it both ways. You can't criticize my PAC contributions while having your own hand out to all the PACs and special interests in the state."

However, Common Cause reported a different story in a study last year. The self-styled citizen lobby said PACs gave Boergers $15,450 from Nov. 19, 1986, to Nov. 1, 1989. During that same period, Common Cause said Shore received $1,925 from PACs.

Common Cause reported this spring that PACs had given more than $1 million to incumbent lawmakers since the 1986 election. Watch for the next letter installment after candidates file up-to-date campaign contribution reports next week. Potter Makes Endorsements

Montgomery County Council member Neal Potter, a candidate in September's primary for county executive, has endorsed three more candidates for County Council, calling them "very well qualified people with a proven commitment."

Potter called a news conference to back Del. Judith C. Toth for District 2; lawyer Derick Berlage for District 5; and former council aide Gail Ewing for an at-large seat.

Potter previously had endorsed council members Bruce T. Adams, Isiah Leggett and Silver Spring activist Gene Lynch for the at-large seats and former school board member Marilyn Praisner for District 4.

Potter said he expected to print a sample ballot listing his name along with his choices for candidates, but said there is no loyalty oath or dues assessment that goes with a slate.

Besides, some of the candidates aren't even supporting Potter in his challenge of Sidney Kramer.

Adams and Leggett are running on a ticket with Kramer, and Ewing said she supports Kramer. Berlage, Toth and Lynch support Potter. Praisner said she is staying neutral in the race.

Potter said he decided against supporting anyone on a "quid pro quo" basis.

"I have simply asked, 'Who has the best qualifications? Who is firmly committed to bringing the county's growth under control and reducing our reliance on property taxes? Who has the independence and strength to make a stand against the kind of bossism that our present county executive has been trying to impose on the council and the Democratic Party?' " he said.

In picking Berlage, Potter bypassed Dianne Smith, a Silver Spring PTA activist who he previously had encouraged to run. Smith was asked to join the incumbents slate headed by Kramer, which at one point included Potter.

Potter said he is not recanting any of the good things he had to say about Smith but on balance thinks the scales tip in favor of Berlage. -- Jo-Ann Armao