In the House, Republicans are favored to hold the chamber thanks, in large part, to their success in gerrymandering districts. Some have suggested Democrats won't be able to win back the majority until the next round of redistricting after the 2020 election.
In the Senate, the GOP also owes its advantage to a kind of gerrymandering. But it's natural gerrymandering rather than the political kind.
To illustrate this point, let's look at the 2012 election. President Obama won 26 of the 50 states -- a bare majority. But he needed to win basically all of the swing states (every one except North Carolina, in fact) in order to win that majority. That's because there are many more solidly red states than solidly blue states, and Democrats need to win the vast majority of the ones in the middle.
In the below chart, we take the 2012 election results in each state and adjust them slightly to show where each state might stand in a completely neutral political environment (that is, rather than Obama winning by four points, he and Romney tied).
Here's that breakdown (with a big tip of the hat to Dave Wasserman's 2012 election results page):
As you can see, there would be 23 states that favored the GOP by double digits and just 13 that favor Democrats by double digits.
Of those 23 double-digit red states, the most vulnerable ones are Georgia, Arizona and Missouri -- not exactly fertile territory for Democrats to compete.
In other words, Republicans, if they just win in the states where they have a clear partisan advantage, should have at least 46 Senate seats -- nearly half the chamber -- in the bag. (More on that later.)
On the Democratic side, the 13 states where the blue team is ahead by double digits give them essentially 26 seats as "gimmes." And even if you throw in nominally blue states like Oregon, Michigan and New Mexico, that gets the party to 32 seats.
So before we even talk about states that should be competitive at the federal level, Republicans have a double-digit advantage, as the map is constructed today.
Democrats, to their credit, have overcome this natural gerrymander in recent years by winning in red states. They have done this thanks to 1) those states' lingering Democratic affections, 2) running conservative Democrats for those seats and 3) by having incumbents who were able to hang on thanks to their incumbency. Democrats have even won some open seats in solidly red states like Indiana and North Dakota, thanks in large part to flawed GOP candidates but also because they ran Democrats who fit the states. As it stands, Democrats actually control 11 of the 46 seats in these solidly red states -- undercutting what should be the GOP's inherent advantage.
As the electoral environment becomes more and more nationalized (and polarized), though, Republicans have a better chance to win back many of these seats. In fact, they can feasibly win six of the 11 back in 2014 alone. And if/when the GOP wins these seats back, it will become harder for Democrats to compete for them and -- by extension -- for the Senate majority.
If Republicans can take over the majority of those 11 seats in the next few elections, Democrats will have to dominate in the swing states just to hold on to a bare majority.
Now, none of this is to say the GOP is guaranteed to take over the Senate eventually. Not hardly. And the good news for Democrats is that a strong majority of swing states do lean slightly blue.
But it's also clear that the House isn't the only chamber in which the map is quite favorable for the GOP's hopes of controlling Congress for years to come.
Update: Here's the Monkey Cage's take on this topic -- a must-read.