A nice win for Republicans today, as Rep. Mike Castle will announce that he's going to compete for the open Senate seat in Delaware. On the Democratic side, Beau Biden is presumably going to run, so this is a real heavyweight bout, at least by Delaware standards. Early polls show Castle with an edge, but I'd call this one a toss-up going in. That's very good for the GOP; if Castle had passed, the seat would be fairly safe for Biden. It is a pickup opportunity for the Dems on the House side. If he wins, Castle would be a strong candidate to become the third most liberal Republican Senator, so that will be worth watching. I've read nothing about the possibility of Castle drawing a conservative opponent in the primary, but he'll have to tread carefully on a variety of votes in the House this year and next, while Biden will be more free to duck tough choices. It's also worth noting that Castle, while very popular in Delaware, will be 71 by the time of the election.
In Florida, meanwhile, the campaign of conservative spoiler Marco Rubio is picking up steam with a decent fundraising quarter. One thing that Crist won't have to worry about is his appointment of fill-in Senator George Lemieux coming back to bite him; as I predicted, LeMieux has been a safely conservative vote so far, sliding in just to the right of Chambliss and Thune to rank as the 10th most conservative Senator so far.
[Udate: typo corrected; thanks Matt!]