Circuit Court Judge William H. Murphy Jr. announced today that he will challenge Mayor William Donald Schaefer in this fall's Democratic primary, saying he believes he can do more for the city's poor as mayor than as a judge, while also implying that Schaefer has not done enough.

Murphy, 40, is the second black to announce a challenge to the three-term incumbent. Murphy made his announcement in a hearing room at City Hall to about 250 backers, many of them black ministers.

Murphy's supporters are hoping for a repeat of last fall's election in which Kurt Schmoke, a young black lawyer, defeated a white two-term incumbent, William Swisher, for prosecutor. Schmoke, given virtually no chance of winning at the outset, united the city's blacks, who make up 55 percent of the population, and crushed Swisher by a 2-to-1 margin in the Democratic primary.

Although Murphy made no direct reference to Schaefer in his brief speech, he said that "in the last 12 years this great city has built marvelous buildings in the center of town which have created the spirit of hope for the rejuvenation of Baltimore. But around those beautiful buildings, in the shadow of affluence, are larger and larger numbers of children without hopes of a decent education, broken families and unemployable parents, hundreds of thousands of functionally illiterate people of all ages, races and creeds, and thousands of people without decent low cost housing."

Schaefer, generally considered the state's most popular politican, never officially announces that he is running for reelection, but has already held a major fund-raiser and made it clear he will seek a fourth term.

Schaefer has been credited with turning around the city's image with his restoration of the harbor area, his aggressive pursuit of federal dollars and with his constant pumping of Baltimore as a place people--and businesses--should want to be. Statewide polls taken early last year showed him as the man most capable of defeating Gov. Harry Hughes, whom the mayor has never gotten along with. But Schaefer opted to stay here, where he is widely considered to be virtually unbeatable.

Murphy defeated a white incumbent when he was elected to his judgeship (which he resigned today) three years ago.

But few politicians here, black or white, see a Schaefer-Murphy contest shaping up along racially polarized lines of the Swisher-Schmoke race.

"Last year the blacks had it all together in the Schmoke race," said one white politician. "That just isn't true here. They're divided. And that means the vote will be divided."

That Murphy does not have unanimous black support was apparent at today's announcement. Although several city council members were present, along with a handful of state legislators, most of the city's black politicians, including Schmoke, stayed away. Already, one of the city's most prominent black politicians, Clarence Mitchell Jr., has come out in support of Schaefer. A group of prominent blacks calling themselves "Blacks For Schaefer" has also endorsed the mayor.

Murphy and his supporters conceded that his race would be difficult and then left City Hall for a prayer service at a nearby church.

"The difference between this and Schmoke-Swisher is that Don Schaefer is a street politician," said Benjamin L. Cardin, Speaker of the House of Delegates, who represents the city's 42nd district. "He isn't vulnerable the way Swisher was because he isn't perceived as being insensitive by the black community. I don't think this race will be as divisive or as emotional as Swisher-Schmoke was."

But Schaefer and Murphy are not friendly rivals by any means. Last year Murphy infuriated the mayor when he ruled against Schaefer's bid to save the city money by reducing garbage collection from twice a week to once. Murphy has often criticized Schaefer for concentrating on rebuilding downtown rather than rebuilding black neighborhoods.

No one disputes Murphy's role as an underdog. Earlier this year, Del. Larry Young (D-Baltimore), one of the city's leading black politicians, predicted it would be 1987 before a black is elected mayor.

"Last year when I had the mayor in my neighborhood, my district, with my own people, the only one they wanted to talk to was him," Young said. "Black people in this city perceive him as being on their side and that means a lot of them will vote for him. Schmoke won because he got all the black votes. Someone running against Schaefer won't have that."

What then motivates Murphy to give up a judgeship to run against a popular incumbent?

"Positioning," said one black politician. "Billy doesn't expect to win, but he does expect to get 40 or 45 percent of the vote. That means a lot of people around the city will know who he is. It will put him ahead of a lot of other blacks if he runs in '87, or if he runs for Congress in the future."

Former Del. Frank M. Conaway, who also is black, and Lawrence Freeman, earlier announced plans to oppose Schaefer, but Murphy is considered more formidable.