This is a guest post by University of California, Davis, political scientist Ben Highton, one of the people behind our House and Senate forecasts.

To fit 100 Senate seats with six-year terms onto the two-year national election cycle, elections for one third – one “class” – of the seats are held every two years.  The class up this year is notably more Republican than the classes not up, providing an advantage to the Republicans as they seek to regain control of the Senate.

Every state has two Senate seats, and each seat is in one of three classes.  California’s seats are in classes 1 and 3.  Alabama’s seats are in classes 2 and 3.  Mississippi’s seats are in classes 1 and 2.  And so on.  Every national election year, one class of seats is up for election.  In 2014, the 33 class 2 seats are up.  (There are also 3 special elections to fill seats that are open from other classes.)

As noted in my earlier post, the Senate treats states as equal – irrespective of population – and this gives the Republicans an advantage because on average, less populous states are more Republican than more populous ones.  What about the states that fall into each of the three Senate classes?  Compared to the national two-party presidential vote margin in 2012, class 2 states are 10 percentage points more Republican on average.  Of the three classes, this is the largest skew toward the Republicans.  The average margin in class 3 states is 6.1 points more Republican than the national presidential margin; and, the average margin in class 1 states is just 1.3 points more Republican.   Here’s a graph showing this:

Moreover, the partisan balance across the states holding special elections in 2014 also tilts Republican.  One of the three is in Hawaii – a strongly Democratic state.  But, the other two are in states that are safely Republican – South Carolina and Oklahoma.

In November, the Democrats have more seats to defend than the Republicans (21 versus 15), and it is a midterm election year with a president from their party whose approval rating is relatively low (although possibly increasing).  Add to those negatives the fact that the states holding Senate elections this year are less Democratic than the states not holding elections, and the Democrats’ 55-45 majority is genuinely at risk.