According to reports, Senator Jon Kyl, the GOP Senate Whip, will not seek reelection next year. A few quick notes...
You're going to read that Gabrielle Giffords would have been a leading, and quite possibly the overwhelming choice, for the Democratic nomination. That's absolutely true. Had she not been attacked, and had she decided to jump in, she would probably have cleared the field. Presumably, no matter how well her rehab goes, she's not going to be able to make the race. Very sad.
Reid Wilson has speculation on some of the other possible names. Kyl was elected in 1994, so it's been a long time since Arizona had an open Senate seat: expect lots of candidates to at least be interested.
This is the fifth retirement announcement already, which seems like an awful lot to me this early in the cycle, but I don't really know that for a fact. I can say that there were fewer than five retirements in five of the last 15 election cycles, and exactly five in a couple more, so we're already more or less at the median number of retirements for recent Senates. So five at this point must be higher than normal, at least.
Arizona appears to be a lean-Republican state, but it's not out of reach for the Democrats, at least in my view. This retirement helps to balance out yesterday's news from Virginia; in my opinion, Arizona is easier for Democrats than Connecticut is for Republicans. Still, on balance the retirements so far clearly help the GOP.