MAYBE CLARENCE THOMAS IS FORGETFUL…. You’d think a sitting Supreme Court justice would be more careful.

Supreme Court Justice Clarence Thomas failed to report his wife’s income from a conservative think tank on financial disclosure forms for at least five years, the watchdog group Common Cause said Friday.

Between 2003 and 2007, Virginia Thomas, a longtime conservative activist, earned $686,589 from the Heritage Foundation, according to a Common Cause review of the foundation’s IRS records. Thomas failed to note the income in his Supreme Court financial disclosure forms for those years, instead checking a box labeled “none” where “spousal noninvestment income” would be disclosed.

Virginia Thomas also has been active in the group Liberty Central, an organization she founded to restore the “founding principles” of limited government and individual liberty.

In his 2009 disclosure, Justice Thomas also reported spousal income as “none.” Common Cause contends that Liberty Central paid Virginia Thomas an unknown salary that year.

In October, there were some interesting questions raised about the propriety of Ginni Thomas collecting “large, unidentified contributions” from unknown sources, including, conceivably, interests with business before the Supreme Court.

But this new wrinkle raises separate questions. While Ginni Thomas engaged in all of this political activism, and received compensation for her work, Clarence Thomas failed altogether to report her income on financial disclosure forms. If he’d done this once or twice, it’d be easier to overlook as some kind of clerical error, but doing so every year from 2003 and 2007 suggests a more deliberate effort.

I’m trying to imagine what the response would be if a similar situation arose with a center-left justice. Imagine if, say, Justice Breyer’s wife considered Republican officials dangerous radicals, and began collecting six-figure checks from secret donors in order to wage a “war against tyranny.” Then, on his financial disclosure forms, Breyer failed to report his wife’s income altogether, despite legal requirements.

The question isn’t whether congressional Republicans would talk openly about his impeachment, but rather, how many congressional Republicans would do so.

Ian Millhiser added that Clarence Thomas isn’t the only conservative justice to play fast and loose with propriety lately: “Justice Antonin Scalia also attended one of Charles Koch’s right-wing fundraising and strategy sessions, and Justice Samuel Alito is a frequent speaker at fundraisers for groups such as the Intercollegiate Studies Institute — the corporate front that funded the rise of Republican dirty trickster James O’Keefe and that used to employ anti-masturbation activist Christine O’Donnell.”

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Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.