Add Jeff Bingaman to the list. He is the sixth senator, and fourth Democratic senator, in a little over a month to announce his intended retirement. With the last one, John Kyl, the Wire wondered what exactly was causing this rash of announcements. Now it seems at least we have an answer as far as the Democrats go; reporting on Bingaman's retirement, The New York Times' Michael Shear writes that "Democratic officials said privately ... that the string of retirements is the result of a concerted push by Senator Patty Murray of Washington, who runs the Senate's campaign committee, to get such decisions made early."

As Ed Morrissey adds at conservative Hot Air, however, an open seat may mean an even harder battle. Democrats may have "hoped to defend" Bingaman's New Mexico seat "with a relatively moderate incumbent." As of a February 8 Public Policy Polling analysis, Bingaman would still have been "a heavy favorite" in the 2012 elections. Instead, this retirement, as The Washington Post's Chris Cillizza puts it, "creates an open seat contest that both national parties will almost certainly target."

Here's what we're wondering, of course: who's next?