A United States magistrate judge in the Southern District of New York certified a class-action lawsuit on Tuesday against the Department of Commerce, which alleges that the Census Bureau’s use of criminal background checks for part-time workers unlawfully screened out 250,000 African-American applicants. The plaintiffs said the bureau gave applicants with criminal records 30 days to provide court records detailing their criminal history, a request they contend is in violation of federal civil-rights protections. Adam Klein, the lead lawyer for the plaintiffs, said that many of them had difficultly providing documentation or had minor charges against them or had been arrested but not convicted of a crime. African Americans and Latinos are also disproportionately arrested compared with whites, Mr. Klein said.