House Democrats called for a Justice Department investigation into whether Supreme Court Justice Clarence Thomas breached ethics rules by failing to report his wife’s financial matters while sitting on the high court.

The plea comes just days after the Supreme Court announced it would hear challenges to President Barack Obama’s health care law, a hallmark of the Democratic agenda that party leaders have charged was wrongly politicized by conservative activists such as the associate justice’s wife, Virginia Thomas.

Virginia Thomas earned as much as $1.6 million since 1997, according to documents obtained by Common Cause and the Alliance for Justice, and a large chunk of that salary came from the conservative group the Heritage Foundation, which vigorously opposed Obama’s health care law.

In September, the group of Democrats, led by Rep. Louise Slaughter (N.Y.), called on the U.S. Judicial Conference, the governing body for federal courts, to look into possible ethical violations related to Thomas’ failure to report his wife’s income. Now, the group of 52 Democrats is calling on the conference to refer the matter to the Justice Department.

In their latest letter to the conference, Democrats suggest that Thomas knowingly omitted his wife’s salary from his disclosure forms and could be in violation of ethics laws that demand a Justice Department probe.

“The new details that have emerged show that Justice Thomas had a firm understanding of the steps necessary to properly disclose all information required by federal law,” Slaughter said in a statement. “It is high time that the appropriate authorities fully investigate this issue to determine if his failure to disclose this information was willful.”

In their latest assault, the group points to information from Common Cause and the Alliance for Justice, which are liberal organizations, that Thomas properly reported his wife’s professional earnings from 1987 to 1997 but stopped after that.

“It is very difficult for Justice Thomas to make a credible argument that he understood the filing instructions for ten years but then misunderstood them for the next thirteen years,” the lawmakers wrote to the conference.

The Supreme Court’s decision to take up the health care law has also spurred some Democrats to call on Thomas to recuse himself, given his wife’s political activity regarding the law. Conservatives in Congress have also called for Justice Elena Kagan to recuse herself from the case because, they say, she was part of the Obama administration when the law was enacted.

Correction: Nov. 18, 2011

An earlier verison of this article misidentified the conservative group that was the source of a large chunk of Virginia Thomas’ salary. It is the Heritage Foundation.