SCANDAL-PLAGUED ENSIGN ANNOUNCES RETIREMENT…. For months, in bold defiance of decency and common sense, scandal-plagued Sen. John Ensign (R) of Nevada swore he would not only seek re-election next year, but was also confident in his chances.

Today, this fantasy came to an abrupt end.

Republican Sen. John Ensign won’t be trying to reclaim his U.S. Senate seat in 2012.

Ensign made the announcement today during a noon news conference at the Lloyd George Federal Courthouse in Las Vegas. He isn’t resigning his seat — simply not pursuing his re-election in 2012.

He said he came to the decision in the last week or so, adding that a Senate Ethics Committee investigation had “zero effect” on the decision not to seek re-election.

“I just came to the conclusion that I couldn’t put my family through this,” he said.

Oh, I see, now he’s worried about putting his family through a difficult ordeal.

To briefly recap for those who’ve forgotten, Ensign’s humiliation came to public attention in June 2009, when we learned the conservative, “family-values” senator carried on a lengthy extra-marital relationship with one of his aides, who happened to be married to another one of his aides. Ensign’s parents tried to pay off the mistress’ family.

The scandal grew far worse when we learned that the Republican senator pushed his political and corporate allies to give lobbying contracts to his mistress’s husband. When Douglas and Cynthia Hampton left Ensign’s employ — because, you know, the senator was sleeping with Cynthia — Ensign allegedly took steps to help them make up the lost income, leaning on corporate associates to hire Douglas as a lobbyist, and ignoring ethics laws that restrict how quickly former aides can begin lobbying careers.

For whatever reason, federal investigators found that Ensign’s wrongdoing didn’t warrant prosecution, but an investigation from the Senate ethics committee, as of just last week, appears to be intensifying, and very likely influenced the senator’s decision.

Ensign’s departure marks the end of what was once a promising career. Indeed, before the scandal, it was widely assumed that Ensign would run for president in 2012, and was considered a credible candidate. Now, faced with the reality that he’d lost voters’ trust, Ensign has no choice but to exit the stage.

Somewhere, Sen. David Vitter (R-La.), who easily won re-election after getting caught with prostitutes, is laughing his ass off.

As for what’s next, ordinarily, when a Republican incumbent steps down in a swing state, this is great news for the DSCC. Not so with Ensign — Dems hoped he’d run, because he’d be easier to beat. As things stand, Rep. Dean Heller (R) is now even more likely to run for the Senate, and will probably be considered the early favorite. Sharron Angle — remember her? — may try another Senate campaign, but the smarter move would be for her to run for Heller’s House seat.

All told, Ensign is the eighth incumbent senator to retire in advance of the 2012 elections. Of the eight, there are four Democrats, three Republicans, and an independent.

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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.