THE JUDD GREGG CATCH…. Sen. Judd Gregg (R-N.H.) confirmed yesterday that he is under consideration to join the Obama administration cabinet as Commerce Secretary. And what about the prospect of Gregg’s departure giving Senate Democrats a 60-seat caucus? There might be a catch.

Sen. Judd Gregg (R-N.H.) won’t accept a position as President Obama’s secretary of Commerce unless he is guaranteed his Senate seat remains in GOP hands, said two Republicans who know Gregg well.

Departing the Senate without one could give Democrats 60 members and a filibuster-proof majority.

“Gregg would never allow his seat to go to a Democrat, the only way he would allow it is if he died,” said a Republican close to Gregg. “He would consider it to be a breach of trust to people who elected it.”

In other words, Gregg would want to strike a deal — he joins the cabinet, New Hampshire Gov. John Lynch (D) would choose a Republican to replace him — before accepting an offer from the president. In fact, rumors were common yesterday that Lynch had his eye on former Gov. Walter Peterson, a liberal Republican, as a leading candidate to succeed Lynch.

I’m curious, though: if Gregg sticks to this, what’s the incentive for Obama to select him? Gregg considers himself an expert on economic and fiscal issues, but he’s still a Republican who backed all of Bush’s economic policies. The president might get some credit for having added a third Republican to his cabinet, but all evidence suggests the congressional GOP isn’t going to negotiate with the White House in good faith anyway, no matter how many gestures Obama makes.

I suppose one possible advantage is a slightly more reliable Republican ally for Obama in the Senate — trading one relatively moderate GOP senator for a dependably moderate GOP senator who’ll step down in 2010 anyway.

Still, all things being equal, if Gregg is applying this condition, the appeal of his nomination goes down considerably.

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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.