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 .