He announced last month that he wouldn’t seek reelection in 2012 but added that he’d finish out his term. Now, suddenly, a change of plans. I wonder who knocked on his door.

Sources confirmed this afternoon that embattled U.S. Sen. John Ensign will resign from office on Friday, opening the door for Republican Gov. Brian Sandoval to appoint Rep. Dean Heller to finish out the term.

If Heller is appointed, it would give him a strategic leg up as an incumbent against his presumed opponent in the 2012 Senate race, Democrat Rep. Shelley Berkley. Berkley is still expected to run.

Either that Senate ethics investigation has taken yet another turn and he expects even more embarrassing revelations are on the way or someone in the GOP made him a lucrative post-Senate offer in order to speed things up and clear the way for Heller. Heller had already announced his candidacy for Ensign’s seat and is currently the favorite in the race, but in a purple state with an open seat and Obama at the top of the ticket, there’s no sense taking chances and waiting until 2012. Same goes for the House seat that Heller will now vacate (and which Sharron Angle has expressed interest in). It’s in a red district, granted, but better to put it on the block now and try to lock in another Republican than to risk a battle over a vacancy after another 18 months of Democratic “Mediscaring.”

The big loser here, notes Dan “Baseball Crank” McLaughlin: Grassroots conservatives who might have been hoping to primary Heller. I think that was a small-ish risk given his sterling poll numbers and the fact that tea partiers seem more attuned to electability now (especially in Nevada!), but the point is well taken. Exit question: Er, does this maneuver of appointing a Senator in order to make his path to election easier actually work? Nate Silver says … nope.

Update: Idle thought: Does the coming debt-ceiling struggle in the Senate and House have something to do with the timing here? Heller’s vote will be under a stronger microscope in the smaller upper chamber, but it might actually matter less than if he was still in the House. Remember that McConnell and other Republican leaders might decide to support cloture on the debt-ceiling vote and then let Democrats own it with a party-line simple-majority vote on the final bill. If that’s what ends up happening (and assuming Reid can find seven Republicans willing to vote for cloture), Heller’s vote won’t matter. In the House, though, every vote will matter. There’s nowhere to hide, even in a crowd of 435.