Other race rankings from Daily Kosthe Hotline, and 538.com

With Mike Castle's announcement that he's running for the Delaware Senate seat vacated by Vice President Biden today, it's possible -- not probable but possible -- that the seats formerly held by Barack Obama (IL -- the incumbent is appointee Roland Burris), Hillary Clinton (NY -- the incumbent is appointee Kristin Gillibrand), Harry Reid and Biden could all be in jeopardy in 2010 ... this is a sign of Republican confidence, solid Republican recruiting (which reflects confidence), and fundraising parity. At the beginning of the cycle, it looked like Democrats would have to defend a few seats and had the chance to pick up even more -- but they're now defending at least as many tough challenges as the Republicans.  How will Republicans exploit this opportunity? By matching the right type of candidate with the right state.
Castle, who first ran for office in 1967, is popular in the state and is quite ambitious, but he is also cautious, having rejected previous entreaties to run. That he'd be willing to risk a career-ending defeat to Beau Biden, the son of the sitting vice president, suggests that national Republicans have promised him significant financial and messaging help, as well as a degree of comfort about his prospects. Castle's entrance makes the race a toss-up. Add to that a slew of Democrats in real trouble -- Sen. Chris Dodd, though he's taking less of a battering in the press, remains in relatively poor shape, as does the majority leader, Sen. Harry Reid, who, although he has yet to attract a top-tier challenger, has seen his approval ratings decline among independents and Republicans, and Democrats could fairly easily lose several seats.

Another state that has the potential to tip: Pennsylvania, where President Obama's approval ratings have dipped below 50%, and where the Democratic primary isn't likely to unify the party. Some primaries are good for the party that survives them; others aren't.

The bright spot for Democrats right now is in Ohio, where President Obama remains relatively popular compared to national Republicans -- this is retiring Sen. George Voinovich's (R) open seat -- and in Louisiana, where Rep. Charlie Melancon (D)'s challenge to Sen. David Vitter (R) is taken seriously, even though, with the migration of black Democrats from New Orleans after Katrina, the state has tilted more Republican. In Missouri, frustration with Republicans is evident, and the likely Democratic nominee, Robin Carnahan, is a sentimental favorite with high approval ratings. It'll be very interesting to see how many times -- if at all, until right at the end -- President Obama campaigns for her.

In New Hampshire, where Sen. Judd Gregg (R) is retiring, independents seem to be swinging away from Democrats; likely nominee Rep. Paul Hodes (D) will find a tougher-than-expected challenger from the state's attorney general, Kelly Ayotte (R). The question: are several cycles' worth of Democratic gains enough to blunt a typical party-in-power off-year performance?

Arguably, the best news for Republicans of the cycle was the retirement of Sen, Jim Bunning, the stinkiest albatross around the neck of Republicans. In a strong GOP year, the Republican nominee will have a distinct advantage over the Democrat, although both Democrats in the race -- AG Jack Conway and LG Dan Mongiardo -- have a good shot if they run their races the right way.

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