Twenty House Democrats called Thursday on the U.S. Judicial Conference to formally request that the U.S. Department of Justice investigate Justice Clarence Thomas's non-compliance with the Ethics in Government Act of 1978.

Justice Thomas indicated on his annual financial disclosure forms that his wife had received no income since he joined the bench in 1991, despite the fact that his wife had in fact earned nearly $700,000 from the Heritage Foundation from 2003 to 2007.

The Ethics in Government Act of 1978 requires Supreme Court justices to disclose their spouse's income.

"To believe that Justice Thomas didn't know how to fill out a basic disclosure form is absurd," Congresswoman Louise Slaughter (D-NY) said. "It is reasonable, in every sense of the word, to believe that a member of the highest court in the land should know how to properly disclose almost $700,000 worth of income."

"To not be able to do so is suspicious, and according to law, requires further investigation. To accept Justice Thomas's explanation without doing the required due diligence would be irresponsible."

The letter (PDF) comes a day after President Barack Obama asked the U.S. Supreme Court to rule on the Patient Protection and Affordable Care Act, his landmark health reforms.

Seventy-four Members of Congress in February signed a letter calling for Justice Thomas to recuse himself from cases involving the Patient Protection and Affordable Care Act because of his family's financial ties to groups dedicated to lobbying against it.

In response, Justice Thomas released his new financial disclosure form in May. It indicated his wife received a $150,000 salary from the group Liberty Central in 2010. The group, which she co-founded, fights to repeal health care reform, among other things.

The appearance of a conflict of interest merits recusal under federal law.

Photo: Flickr user Phil Roeder.