Nevada Senator John Ensign today disclosed that he'd had an extramarital affair with a campaign staffer. This will make for plenty of interesting water-cooler gossip, particularly since Ensign has a penchant for calling on people to resign for various and sundry moral and ethical lapses -- notably Larry Craig, Bill Clinton and Ted Stevens (but not David Vitter).
It seems unlikely, however, that Ensign will resign himself. Although Nevada's governor is a Republican and could appoint another Republican to replace him, that would nevertheless trigger a special election in 2010, when Democratic incumbent Harry Reid is also on the ballot. Nevada Republicans have a very poor bench right now and are already having trouble recruiting a credible candidate to run against the unpopular Reid. They might have a lot of difficulty retaining Ensign's seat in the event of a special election, or alternatively, might compete for it at the price of giving Reid a free pass.
Remember, senators don't have to govern, or to preside over any legislature. They don't have any particular use for political capital, and other than their ability to be re-elected, they don't have any particular reason to popular. That's why Eliot Spitzer resigned and David Vitter (whom many Louisanans seem to have forgiven) didn't. It's why Roland Burris is still in the Senate.
Still, whether Ensign runs for re-election or not, this certainly would seem to give the Democrats a leg up on the seat in 2012, a cycle in which they'll have few other opportunities to play offense as they try to defend the gains they made in 2006. And Ensign's is a valuable seat at that: only one state more Democratic than Nevada currently has a Republican senator. That's Maine, which has two of them. But whereas Susan Collins and Olympia Snowe are moderates, Ensign is a staunch conservative who chairs the Senate Republican Policy Committee.