Divided government, Parte Deux (2012 Senate overview, with spreadsheet)

It's not exactly news that elections can be weird, but when it comes to U.S. Senate "classes," it's the norm. This has to do with Senators' six-year terms and the ups and downs of presidential and mid-term election cycles. 

Here's why: Only 33 of the 100 Senate seats are up for re-election in 2012, and because of those presidential/midterm cycles, there's a distinct rhythm to these things. This is Class I, which picked up eight seats in the notable 1994 Republican Revolution election. This meant that when it came time for Class I to stand election again in 2000, Republicans were defending more seats, and lost four of them, knotting the Senate at 50-50. Then in 2006, the Democrats went on a mid-term tear and gained six seats and 51-49 control of the Senate.

Which brings us to today. Heading into 2012, Class I consists of 23 Democrats and 10 Republicans. From a political science perspective, that means Democrats are on the defensive, because a 23-10 majority means there just aren't that many seats to be gained, and lots of seats to protect .