A DEMOCRAT’S ETHICS PROBE VS. A REPUBLICAN’S CRIMINAL PROBE…. If the accounts from major media outlets are any indication, the political world is awfully excited about the ethics allegations against Rep. Charlie Rangel (D-N.Y.). To be sure, the interest is warranted — the allegations against the former Ways and Means Committee chairman are serious; Republicans are thrilled; and the controversy has literally become front-page, above-the-fold news.

There may be some rule that I’m not aware of, prohibiting coverage of Republican scandals, but while a House Democrat’s ethics problems intensify, a sitting Republican senator is still the subject of an ongoing criminal investigation, which is also getting more serious.

The Senate on Thursday night quietly approved a resolution that will allow Sen. John Ensign’s aides to testify to a federal grand jury investigating the aftermath of the Nevada Republican’s extramarital affair with a former campaign aide.

By voice vote, the Senate approved the resolution that would authorize employees of the Senate to give testimony to a grand jury in Washington.

Senate aides said that the resolution was necessary because Senate rules would prohibit employees from testifying outside of the halls of Congress.

Politico added that the move, which nearly every major outlet ignored, “is the latest sign that the investigation … continues to move swiftly.”

This development comes just a week after Sen. Tom Coburn (R-Okla.), a former Ensign housemate, announced that he’d agreed to cooperate with the federal criminal investigation surrounding the conservative Nevadan. Coburn turned over more than 1,200 pages of documents to the Justice Department, including emails from Ensign.

And that development came on the heels of news that Ensign’s aides have told investigators that the senator knew he was violating ethics rules on lobbying restrictions, but did it anyway.

As a rule, when a high-profile U.S. senator is facing a criminal investigation, the media shows at least some interest. When that investigation involves sex, the media tends to show quite a bit of interest.

But for reasons I still can’t explain the Republican Nevadan is getting a pass. Here we have John Ensign, a “family values” conservative Republican, who had an extra-marital sexual relationship with his friend’s wife, while condemning others’ moral failings. Ensign’s parents offered to pay hush-money. He ignored ethics laws and tried to use his office to arrange lobbying jobs for his mistress’ husband. The likelihood of Ensign being indicted seems fairly high.

And yet, there’s no media frenzy. No reporters staked out in front of Ensign’s home. No op-eds speculating about the need for Ensign to resign in disgrace. Instead, the media’s fascinated with Charlie Rangel.

Rangel is facing a probe from the House ethics committee, while Ensign is under scrutiny from the FBI.

Is this just the IOKIYAR rule taken to the extreme? Was there some kind of memo stating that only Democratic scandals deserve media attention in an election year?

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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.