Why did Republicans take back the Senate? It's the map, stupid. Sometimes -- ok, a LOT of the time -- people like me make politics more complicated than it really is. Gallons of (electronic) ink are spilled on every moment of a Senate race, looking for meaning and momentum. But, in the end, politics is often very, very simple. Like the battle for Senate control on Tuesday. The biggest reason Republicans won back control of the Senate was the seats that were up in 2014 heavily favored their side. Seven Democratic-held seats were in states Mitt Romney carried in 2012; Republicans flipped six of them with the seventh, in Louisiana, headed to a runoff on Dec. 6 where the GOP is favored. This chart -- from Fix friend and Republican lobbyist Bruce Mehlman whose PowerPoints are the stuff of legend in D.C. -- shows that Republicans won largely because of the ground on which they were fighting. Those numbers make clear just how disastrously bad the map was for Democrats. But they may even underplay the built-in difficulties Democrats had going into election day because the chart doesn't take into account the retirements of well-established (and totally re-electable) Democratic Senators in Montana, South Dakota and West Virginia. That trio of retirements made those seats virtually un-holdable. The difficulty of the terrain was compounded by President Obama's declining popularity in virtually every one of the states where Democrats were trying to hold on. In places like Kentucky and Arkansas, Obama's approval rating was in the low 30s. In North Carolina, the high 30s. Even in states like Iowa and Colorado, which Obama won in 2008 and 2012, his numbers weren't anything great. Less than four in ten Iowans approved of the job Obama was doing, according to the exit poll. Republicans won states where Republicans traditionally win. That's how they won the majority. Even without winning Iowa and Colorado, Republicans would have the six seats they need to be in the majority assuming Alaska falls their way, which it should. The map is, of course, cyclical. So, in 2016, Republicans will be the ones faced with the tough terrain; there are 23 Republican seats up compared to just 10 for Democrats and in five of them -- Illinois, New Hampshire, Ohio, Pennsylvania and Wisconsin -- Obama won in 2008 and 2012. If Democrats manage to take the Senate back in two years time, they'll have the map to thank. Just like Republicans in 2014.