Democrats are probably going to lose control of the Senate. One reason is that voters are genuinely unhappy with what they see in Washington—and, rightly or wrongly, they blame President Obama and his allies for what’s going on. But that’s not the only reason Democratic control of the Senate is in jeopardy. Another, potentially bigger reason is geography. Senators serve staggered, six-year terms. And it so happens that the states with Senate elections this year are disproportionately conservative.

How do we know this? One way is by looking at how those states voted in 2012, the most recent presidential election year.

In the actual election that took place, with all 50 states plus the District of Columbia voting, Obama won handily over Mitt Romney. Obama got 332 electoral votes, while Romney got just 206. But if the electorate in 2012 had consisted only of voters living in states participating in this year’s Senate elections, Romney would have won comfortably, with 165 electoral votes to Obama’s 130.

It’s not hard to see why. Texas has a Senate election this year. Georgia, North Carolina, and Tennessee do, too. But California, Florida, New York, Ohio, and Pennsylvania—all of which Obama won in 2012—don’t have races this year.

Another, more scientific analysis comes from Patrick Egan, of The Washington Post's Monkey Cage. Egan did a similar analysis, only he zeroed in on the popular vote of the states with Senate races. And then he compared this year's Senate races to previous ones. “In the typical Senate election state, Republican presidential nominee Mitt Romney ran ahead of his national margin by more than seven percentage points,” Egan writes. “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.”

Source: The Monkey Cage

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.