This is the scariest sentence for Senate Democrats going into Tuesday's vote: The 2014 Senate electorate leans about 7.2 points more Republican than the country as a whole.
The set of Senate seats up for reelection this year is particularly concentrated in Republican-leaning states. That's a recurrent phenomenon in American politics, as this chart from NYU political scientist Patrick Egan at the Monkey Cage shows:
Because only about a third of the Senate is up in any given election, experts divide Senate seats into three separate "classes." Class 1 seats were up in 2012. Class 2 seats are up this year. And Class 3 seats will be up in 2016. The states with Class 2 seats, as the above chart demonstrates, have a strong Republican lean. Class 1 seats have a slight Democratic lean, but it's much smaller, owing to the Senate's overall preferences for small states, which tend to tilt Republican.