Alex Williams' first job after graduating from Howard University Law School 13 years ago was as a law clerk in the office of Prince George's State's Attorney Arthur A. Marshall.

Today, the Howard University law professor has decided to go after another job in that office -- the top one.

Williams, 37, who is also vice chairman of the Washington Suburban Sanitary Commission, plans to announce Sunday that he's running for state's attorney in the September Democratic primary. Williams would be the first black Democratic candidate for a countywide office in Prince George's County, said Bianca Floyd, a black history specialist with the Maryland-National Capital Park and Planning Commission.

"This will be a first for us," said Bennie Thayer, a county businessman who heads the Prince George's Rainbow Coalition: "a black candidate on a countywide level who has a strong possibility of winning."

Marshall, a fixture in county government since his election in 1962, faced his most recent primary opponent in 1970.

Williams said several events led to his decision to run against Marshall, who, by all accounts, will be difficult to unseat.

The recent election victories of Virginia Lt. Gov. Douglas Wilder and Boston City Council Chairman Bruce Bolling were encouraging, Williams said, because each was the first black to win his office.

Also, presidential candidate Jesse Jackson's impressive showing in the county in the 1984 Maryland primary was an indication of the growing power of Prince George's black voters, Williams said. Jackson won 44 percent of the vote against seven opponents in the county.

But an even more important factor, Williams said, was Marshall's decision to run for two circuit court judgeships in 1984. Marshall lost to judges Arthur M. Ahalt and G.R. Hovey Johnson by several thousand votes.

"I don't make decisions with haste," Williams said in an interview last week. "I have been thinking about running for this office a long time. Twenty-four years is an awfully long time to be in the same job. I think the zeal, zest and fire that (Marshall) had years ago are gone."

Marshall, who in December announced his plans to seek a seventh consecutive term as state's attorney, said in 1983 of his decision to try to switch jobs from prosecutor to judge that "it's time." Three years later, the 55-year-old Democrat still wants to be a judge. But Marshall said he will not treat his seventh campaign for state's attorney lightly.

"I run every campaign with a great deal of zeal," said Marshall, whose first fund-raiser is scheduled for the end of this month.

Some county politicians said Marshall's unsuccessful judicial campaign and Prince George's growing black electorate -- the state estimates that the county will be more than half black by July 1 -- won't be enough by themselves to carry Williams to victory. No opponent has come close to Marshall in the general election since 1962, and Marshall could not even remember the names of his latest opponents.

"Alex Williams is not going to be able to just run on the phenonmenon of Doug Wilder," said Del. Albert Wynn (D-Prince George's). "He is going to have to get out in the county, all the way to the precinct level, to let people see how he compares philosophically with Bud (Marshall)."

Williams, who was the first black lawyer in the county's public defender's office, said he doesn't want race to be an issue in the campaign, although he acknowledges the symbolic importance of being the first black to seek a countywide office.

"I want the voters to realize that this is not a black versus white issue, and to make a decision about who is best able to lead this county into the next decade based on the issues and qualifications," he said.

Williams has served on the county Human Relations Commission and the state Commission on Medical Discipline. He is the town attorney for Glenarden and Fairmount Heights and is on the board of directors of the Greater Laurel-Beltsville Hospital. Williams also is a former juvenile court master, teaches criminal law and has a private practice in the county.

For the moment, Democratic leaders are staying neutral about the race for state's attorney. The filing deadline for the September primary is June 30.

Thayer, of the Rainbow Coalition, said Williams' candidacy will give party leaders in the county a chance "to reach into the black community." Wynn talks about an opportunity for "power sharing" among Democrats.

"I have voted for Marshall in the past because I'm a Democrat," said Alfonso N. Pearson, a county lawyer who previously worked as an assistant state's attorney under Marshall. "When he ran for judge, I did support him. Right now, I think Alex is a good candidate for state's attorney. But I'm not throwing roses to Bud, or to Alex, either."