Conservative Republicans are capitalizing on the troubles of community activist group ACORN—ranging from charges of voter registration fraud to embarrassing videos of its employees—to revive their long-standing fight against a federal law that grades banks on their investments in poor and minority neighborhoods. The 1977 Community Reinvestment Act was intended to end redlining, a practice in which banks in effect walled off many inner-city neighborhoods from mortgage loans. But some GOP lawmakers say it has outlived its purpose and is being used inappropriately by ACORN to shake down banks for money. They want to repeal the law, scale it back...