Maryland Attorney General Stephen H. Sachs was showing off the partially healed blister on the third finger of his right hand last night, a blister that he got shaking hands at a friend's fund-raiser in Baltimore a month ago.

Then he turned around and began shaking hands again, this time with the more than 250 people who attended his first Washington area campaign rally, held at Prince George's Community College in Largo.

The purpose of the free rally, which featured blue and yellow balloons, hand-lettered campaign placards and neon-yellow bumper stickers, was to sell Sachs to the masses in suburban Prince George's County as the most logical candidate for governor of Maryland in 1986.

In a short speech, Sachs stressed the importance of the Prince George's vote and mentioned the enhancement of the University of Maryland at College Park and a continued upswing in economic development as two of the objectives he will emphasize during his campaign. He said afterward that he feels his biggest competition for votes in Prince George's County during the months ahead will be Baltimore Mayor William Donald Schaefer, one of his likely rivals for the Democratic gubernatorial nomination. Another is House of Delegates Speaker Benjamin L. Cardin.

"I'm anxious not to appear too cocky," he said as he mixed with the crowd, which was munching popcorn and drinking tap beer. "But I'm telling you right now I'm going to carry Prince George's County, no matter who" else is running.

Sachs arrived at the rally from Annapolis, where he has been involved in the continuing crisis surrounding Maryland's savings and loan industry. "Except for the Preakness, this was the only thing on my schedule I've kept in the last three weeks," he said.

Last night's event attracted a handful of state delegates, County Council members and a host of volunteer political activists.

Some, such as Maryland Secretary of State Lorraine Sheehan, are diehard Sachs supporters who have declared their support for his candidacy.

Others, like R. Dan Ritchie, a Republican, and Linwood Jones, president of the county's Black Democratic Council, said they were merely there to "look him over."

But many, like Bennie Thayer, chairman of the Maryland Rainbow Coalition, were there because they would not miss such a party. "There's no one else running at this point," said Thayer, who admitted to leaning in Sachs' direction. "Obviously there are people here who wouldn't be here if there were other people in the race." He added, "It would be nice to see how this would go next October or November."