The Senate must not be the fun-loving, free-wheeling place it always was. Jeff Bingaman, the Chair of the Senate Energy and Natural Resources Committee, will announce his retirement later today, according to sources. He becomes the sixth Senator – three Democrats, two Republicans, and Joe Lieberman – to announce their retirements in the past month.

Bingaman had been mulling whether to run for a fifth term for months and, if he had, would have almost certainly been re-elected. He told Senate Majority Leader Harry Reid of his decision to retire last night.

His retirement, however, creates an open seat contest that both national parties will almost certainly target. Democrats should start the race with an edge, however, given President Obama’s 15-point victory margin in the state in 2008.

The last time there was an open Senate seat in New Mexico, in 2008, former Rep. Heather Wilson, seen as a moderate (I’m not sure why), got an early taste of the power of the Tea Party, getting beat by far-right conservative Steve Pearce. This made the race instantly uncompetitive, and Tom Udall cruised to a 26 point win. Pearce ended up getting his job back in the House in 2010; Wilson didn’t. But the exact same thing could happen again, with Pearce and Wilson locking in a death struggle, Pearce winning, then finding he’s unelectable statewide. Reps. Martin Heinrich and Ben Ray Lujan would be possible candidates for the Democrats, along with former Lt. Governor Diane Denish.

The larger point is this. Thirty-three Senate seats are up for grabs in the next election. Six of them are now open seats. Combine that with the turnover of the last three cycles, and a substantial portion of the US Senate in 2013 will be composed of people who were not around in 2006. At some level, that’s progress. At another, it shows that a dysfunctional legislative body isn’t all that joyous to its members, even despite the substantial residual benefits that go along with it.