Representatives of six Mexican-American groups pledged their support today for Herman Sillas Jr., the embattled U.S. attorney for the northern district of California, whose firing is being considered by the White House.

Implicit in the news conference was a threat to deny President Carter Mexican-American support in California if Silas is fired.

The Washington Post reported Sunday that Sillas failed two lie-detector tests concerning allegations that he received $7,500 in bribes from Richard Timothy Workman, a convicted southern California auto dealer, just before Silias became director of the California Department of Motor Vehicles in 1975.

Sillas allegedly took the money in return for a pledge to return Workman's auto dealer license. But after he became DMV director, Sillas allegedly was unable to do anything for Workman. Workman, who passed three polygraph tests concerning the allegations, said Sillas did not return the money.

Despite a request by the Justice Department, Sillas has refused to resign. A White House source told The Post it had been hoped that Sillas would resign an spare the administration the embarrasment of firing a prominent Hispanic leader.

The administration's fears about political problems stemming from the situation appeared justified from the comments here today.

"If Carter wants to wrongfully take such an unprecedented action [as firing Sillas], then Carter will suffer at the polls, and the Hispanic community will work to make this blunder known at election time," said Gaspar Olivedo of Sacramento Concilio, an umbrella group of 21 Sacramento-based Chicano organizations.

Others at the news conference were La Raza Lawyers Association of California, the American G. I. Forum, IMAGE de California, the Mexican-American Chamber of Commerce, the California Coalicion of Hispanic Organizations and the California Democratic Caucus.

The groups are organizing a telegram campaign to support Sillias' retention.