The legislators requested an investigation in a letter Thursday to the Judicial Conference, The Hill reported. They say Thomas didn't reveal in disclosure forms his wife's income from groups opposed to U.S. President Barack Obama's healthcare law.
Actions by the Obama administration and 26 states that want the law overturned may have the U.S. Supreme Court hearing the case in the upcoming term. That would most likely mean a decision during next year's presidential campaign.
A group of Democrats in February requested that Thomas take himself off the case. U.S. Sen. Orrin Hatch, R-Utah, made the same request of Justice Elena Kagan, who was Obama's solicitor general when the law was being written.
Many recent controversial cases have been decided by 5-4 votes with Justice Anthony Kennedy siding sometimes with the conservative bloc and sometimes with the liberals.