Rep. Parren J. Mitchell, the political leader of the state's black community, joined the Democratic gubernatorial ticket of Attorney General Stephen H. Sachs today, a move that added zest and the possibility of wider appeal to Sachs' underdog campaign.

In festive news conferences here and in Prince George's County, Mitchell and Sachs clasped hands in a symbolic gesture that underscored their philosophical compatibility in pursuit of the campaign's "One Maryland" theme and that buoyed the hopes of Sachs' supporters for overtaking Baltimore Mayor William Donald Schaefer's commanding 2-to-1 lead in the polls.

Calling Mitchell a "richly qualified man of government," Sachs said he had wooed the 64-year-old retiring dean of the Maryland congressional delegation and civil rights spokesman as his candidate for lieutenant governor because "of the values he and I share."

"Parren has spent a lifetime in pursuit of the cause of equality and the cause of opportunity," Sachs said. "Parren Mitchell does not trim his conscience to fit this year's fashion and neither do I."

Supporters of the attorney general's candidacy called the addition of Mitchell a stunning coup that would immediately boost Sachs' appeal among blacks, who make up almost one third of the Democratic primary vote and who have been supportive of the mayor in past races. They also asserted that the ticket would focus attention on the ideological and stylistic contrasts with Schaefer.

Others, principally Schaefer's supporters, predicted that the elevation of a liberal black to the ticket would cause a backlash among conservative whites in suburban Baltimore and would drive moderate and conservative Jewish voters to rally behind the mayor.

In a statement released through his campaign spokesman, Schaefer, who has yet to announce a running mate, said his own campaign would "never narrow its appeal to selected segments of Marylanders," a characterization of the Mitchell choice that drew immediate counter-criticism from Sachs.

"That's a gross and regrettable attitude," Sachs said. "It shows again that the mayor is only a man of the past . . . and that he looks at one of Maryland's most talented and qualified public servants and all he can see is the color of his skin."

Mitchell lost no time in assuming a role for which Sachs' supporters believe he is uniquely qualified: calling into question the Baltimore renaissance that is the heart of Schaefer's campaign.

"Many of you come over to Baltimore to see that beautiful harbor," said Mitchell, who has sometimes been an outspoken critic of Schaefer's priorities in the majority black city. "The next time you come over, walk 30 blocks in either direction of that harbor and you will see people living in poverty . . . . One Maryland means that everybody's life, everybody's environment should be as beautiful and as safe and attractive as the Inner Harbor."

Mitchell's decision, two weeks after he reaffirmed his retirement from Congress after 16 years, has electrified supporters of Sachs' bid for governor, caught Schaefer backers momentarily off guard, and begun to alter the dynamics of local politics, particularly in Prince George's County.

Despite months of intensive campaigning, Sachs has had only limited success in eroding Schaefer's strength among blacks, according to most polls. Mitchell gives Sachs an entree with black voters who have historically given strong support to Schaefer but who revere the Baltimore congressman because of his family's long ties to the civil rights struggle. His supporters say Mitchell has the ability to galvanize not only the state's black political leadership, but also rank and file black voters.

Noel Myricks, a 50-year-old University of Maryland professor who described himself as "leaning to Schaefer" before today's events, illustrates the hopes of the Sachs-Mitchell campaign. "I could never vote against Parren," Myricks said today at Sachs' news conference.

"Parren is perhaps the most popular black elected official ever in the state," said state Del. Larry Young, a black legislator from Baltimore.

The possible impact of Mitchell's candidacy on black elected officials was seen today when state Sen. Decatur Trotter attended Sachs' second news conference. Trotter, who owes his position to the dominant Prince George's political organization whose leaders back Schaefer and who is facing an opponent backing Sachs, said that "Parren's involvement would shed new light on the subject and is something we will have to evaluate."

Even in 1983, when Schaefer faced a black opponent, William H. Murphy Jr., the mayor had strong support among black elected officials. But as one Schaefer partisan, who asked to remain anonymous, said last week, "It's one thing to buck Billy Murphy, it's another to buck Parren."

Schaefer supporters assert that Mitchell, for all his stature, will not alter the outcome of the race. Mitchell, after all, was a chairman of Murphy's mayoral campaign, they explain, and the mayor won in a landslide.

"It was the best move Sachs could make under the circumstances, an adroit, skilled move, but I feel ultimately it won't make a difference," predicted Bruce Bereano, a prominent Annapolis lobbyist and fund-raiser for Schaefer.

Other Schaefer backers predict that Mitchell, because he is black, will increase the mayor's margin among conservative white voters, particularly in areas like Baltimore County. And some suggest that Mitchell may be viewed as a divisive influence by Jewish voters who will recall that Mitchell was first elected to the House by narrowly defeating a Jewish congressman.

State Senate President Melvin A. Steinberg (D-Baltimore County), a supporter of Schaefer, called Mitchell a "controversial" figure with a reputation that is "confrontational, militant."

Among Sachs' supporters, though, it is an article of faith that Mitchell can forcefully make the attorney general's case that Schaefer has created "two Baltimores," one for the developers and tourists, and one for the poor and powerless.

"When Parren talks about schools it will be right in the mayor's gut," said Del. Albert Wynn (D-Prince George's). Wynn and other Prince George's County supporters of Sachs believe that the Mitchell candidacy has already tipped the scales in their county, which is nearly 50 percent black, and permanently disrupted the ability of the predominant Democratic organization to enforce discipline on other officeholders.

Secretary of State Lorraine Sheehan, a Prince George's politician who will serve as a statewide coordinator for Sachs, said today that "a week ago we thought we had broken the organization's arm" by keeping some slates neutral in the governor's race. "Now," she said, "I think we've broken another part of their anatomy."