A government watchdog group is asking the Justice Department to investigate what it calls a potential conflict of interest by two Supreme Court justices in a landmark case concerning corporate and union spending on political campaigns.

In a letter sent Thursday to Attorney General Eric HolderEric Himpton HolderOne quick asylum fix: How Garland can help domestic violence survivors First redistricting lawsuits filed by Democratic group On The Trail: Census data kicks off the biggest redistricting fight in American history MORE, Common Cause President Bob Edgar said Justices Antonin Scalia and Clarence Thomas attended "political strategy sessions" hosted by Koch Industries, raising questions about their impartiality in the Citizens United vs. Federal Election Commission case.

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The owners of Koch Industries have regularly made large contributions to Republican campaigns and conservative causes.

The 5-4 court ruling in favor of Citizens United a year ago overturned a ban that ended restrictions on corporations’ and unions’ political spending, which the group said allowed Koch Industries and others to spend at will in the 2010 midterms.

Common Cause argues that the “apparent involvement” of Scalia and Thomas in Koch’s strategy sessions constitutes a conflict of interest, and “if sufficient grounds for disqualification of either Justice exist,” the Justice Department should ask the Supreme Court to vacate the ruling.

Both Scalia and Thomas voted in favor of overturning the ban.

"Given that the facts supporting an appearance of bias were not publicly known at the time of the decision, the appropriate remedy for either violation is for the Department to file a motion with the Court to vacate the judgment," Edgar said.

Common Cause also weighed in on Thomas's wife's position as the head of Liberty Central, a conservative advocacy group. It said that too represents a conflict of interest and was cause for Justice Thomas to recuse himself from the case.

"With respect to Justice Thomas, there may also be an undisclosed financial conflict of interest due to his wife’s role as CEO of Liberty Central, a 501(c)(4) organization that stood to benefit from the decision and played an active role in the 2010 elections," Edgar said.