BALTIMORE, OCT. 27 -- Federal prosecutors lambasted Maryland political figures Clarence and Michael Mitchell today as corrupt operators who helped the scandal-tinged Wedtech Corp. "grease the wheels" to block a congressional investigation of the firm.

In opening statements at the trial of the Mitchell brothers, Assistant U.S. Attorney Gary P. Jordan told a freshly picked jury of seven women and five men that Wedtech's history is a "saga of greed and corruption" in which its officers paid the Mitchells $110,000 not only to block the investigation headed by their uncle, former Maryland representative Parren J. Mitchell, but also to smear a Wedtech competitor in a grab for a lucrative government contract.

Defense attorneys counterattacked, challenging the credibility of key government witnesses, many of them former Wedtech officials already convicted in a separate influence-peddling Wedtech case in New York.

Attorney Abbe D. Lowell described the witnesses variously as ex-cons and thieves whose credibility is in question because they negotiated plea agreements to testify for federal prosecutors in exchange for possibily lenient sentences.

"Do you think they came here out of altruism or because they have a plea agreement tucked in their pocket?" Lowell asked the jury.

The Mitchells are charged with accepting $110,000 in payoffs from Wedtech in 1984 and 1985 to block the House Small Business Committee from looking into allegations that Wedtech used the influence of high-ranking White House officials, including Attorney General Edwin Meese III, to maintain its preferential minority contractor status in doing business with the government.

Clarence M. Mitchell III, a former Maryland state senator, and his brother, Michael B. Mitchell, a former Baltimore City Council member and currently a state senator, have denied the charges. They acknowledge receiving the money from Wedtech but contend it was part of a legitimate retainer to represent the company in legal matters. They have contended that the trial is part of a racist vendetta against civil rights leaders.

In arguing today to the jury of three blacks and nine whites, Lowell contended that business firms lobby and pressure Congress routinely.

"People every day try to stop investigations . . . try to start investigations," he said. "The question is how."

He said the Mitchell brothers were approached by Wedtech officials for help and entered into a formal agreement without knowing of the company's unsavory past.

As further evidence of the Mitchells' above-board relationship with Wedtech, Lowell said, all payments to them were by check, "no cash, no slush funds . . . . The Mitchells did everything they could to create a paper trail."

Lowell acknowledged that the Mitchells agreed to use their influence to get a Wedtech competitor "out of the way" in an attempt to get a large contract to make special chemical decontamination units for the Army. But he denied they put any pressure on Parren Mitchell, their uncle, to stop the investigation of Wedtech.

"They never spoke a word to Parren . . . despite numerous opportunities . . . at Thanksgiving dinner and political rallies," Lowell said.

He said Wedtech may have had the Mitchells in mind as an "insurance policy" in stopping the investigation, but that never became necessary. Instead, Peter Neglia, a former Small Business Administration official in New York described by prosecutors as Wedtech's "man in the SBA" concocted a letter to Parren Mitchell assuring him Wedtech had violated no minority contracting rules and got the investigation called off.

The trial before Judge Norman P. Ramsey is expected to last four to five weeks.