Patrick J. Egan illustrates the advantage Republicans have in this year’s Senate races:
A good measure of the parties’ relative strength in the states holding Senate elections is the share of the state vote each earned in the most recent presidential election. The figure above plots (in blue) the Democratic share of the two-party presidential vote in the median state of those holding Senate elections from 1950 through 2014. For comparison, it also displays (in gray) the share of the national popular vote the Democrats received in the most recent presidential election. For most of the past six decades, these two trends tracked each other very closely: the parties’ relative strength in the set of Senate seats up for election was no different from their strength nationally. But that changed after the 2000 presidential election, in which the Republican Party’s dominance in the South emerged in full force.
His bottom line:
Simply put, this year’s Senate elections are unrepresentative of the nation to an extent that is unprecedented in elections held in the post-war era. So when we begin to sift through the results on Election Night, the number of Senate seats won and lost will tell us less than we might like about where the two parties stand in the minds of American voters.
Jonathan Cohn adds a qualifier:
Of course, it’s not simply geography that’s undermining Democratic strength this time around. If it were, Democrats wouldn’t be struggling to hold seats in places like Iowa, which Obama won. But the electorate for this Senate race is a lot more conservative than America as a whole. That has surely made a huge, and maybe decisive, difference.
Ben Highton highlighted the Republicans’ structural advantages back in February:
[T]he 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:
The Dish 