BALTIMORE -- In a politically and racially charged atmosphere, former Maryland state senator Clarence M. Mitchell III and his brother, state Sen. Michael B. Mitchell, go on trial in federal court today, accused of accepting payoffs to try to block a congressional investigation into the scandal-racked Wedtech Corp.

Jury selection begins today before U.S. District Judge Norman P. Ramsey. The trial is expected to last three to five weeks.

Scions of one of Maryland's most influential black political families, the Mitchells have steadfastly maintained their innocence, claiming the Reagan administration is pursuing them to deflect attention from the past involvement of U.S. Attorney General Edwin Meese III with Wedtech.

More important, they have said in the months leading up to today's trial, there is a "Republican conspiracy" to discredit black leadership in the country. They contend that Meese, as the administration's chief law officer, has been a key figure in directing federal prosecutors to act against the Mitchells and other influential blacks.

Prosecutors here have denied the accusations, and Judge Ramsey ruled in a pretrial hearing in August that the Mitchell investigation was a "home-grown" case developed by Baltimore prosecutors with little or no direction from the Justice Department in Washington.

Neither chief prosecutor Gary P. Jordan nor defense attorney Abbe D. Lowell has disclosed details of their trial strategies, and it was not clear whether the trial will be confined to the specific charges against the Mitchells or broadened to include the political and policy implications alleged by the Mitchells.

In a 14-page indictment, the Mitchells are charged with accepting $110,000 from the New York-based Wedtech Corp. in 1984 and 1985 to block an investigation of the company by the House Small Business Committee, then headed by their uncle, former representative Parren J. Mitchell (D-Md.).

The committee was looking into whether Wedtech, a minority defense contracting firm, received preferential treatment because of intervention by White House officials, including Meese, then a White House counselor.

Meese himself has come under investigation for his alleged role in the case by independent counsel James C. McKay.

Maryland U.S. Attorney Breckinridge L. Willcox has said that Parren Mitchell was not involved in the alleged attempts by Wedtech to block the congressional probe and was a "unwitting victim" of his nephews.

In the indictment, the Mitchells are charged with meeting with various Wedtech officials and accepting three checks totaling $110,000 to block the probe. The checks were paid to the Mitchell family law firm, of which Michael Mitchell, 42, is a member, according to the indictment. Michael Mitchell, then a Baltimore City Council member, in turn gave $25,000 to Clarence Mitchell, 48, who is not a lawyer, and $33,332 to Wedtech consultant Anthony Loscalzo as a kickback for steering Wedtech's business to the Mitchells, the indictment said.

On Friday, Richard Strum, a former high-ranking officer of Wedtech, pleaded guilty to conspiracy in the alleged scheme to block the congressional investigation and is expected to testify in the Mitchell trial.

At Strum's hearing, prosecutor Jordan read a formal statement of facts that said that when Strum approached the Mitchells, Michael Mitchell assured him that if "a large sum of money was paid by Wedtech, the investigation could be stopped." The checks totaling $110,000 then were paid to the Mitchell law firm "under the guise of a legal retainer," according to the statement of facts.

Defense attorney Lowell has ackowledged in interviews that some fees were paid by Wedtech to the Mitchell law firm but said they were for legitimate services.

"The government . . . is accusing the Mitchells of trying to influence a congressional committee," Lowell told a reporter in April. "People do that every day in Washington . . . . Lawyers are paid every day in Washington to stop investigations . . . and I don't see the difference here."

The Mitchell brothers stem from a well-established Baltimore family known by some as the "black Kennedys" of Maryland. Their father is Clarence M. Mitchell Jr., for many years a Washington lobbyist for the NAACP, who died in 1984. Their mother, Juanita Jackson Mitchell, was the first black woman to graduate from the University of Maryland School of Law. A longtime member of the Maryland legislature, Clarence Mitchell III last year ran unsuccessfully for Congress. Michael Mitchell was elected to his Senate seat.