First, the Senate gives equal representation to states, allocating two seats to each state regardless of its population. To see why this matters, consider the 2012 presidential election. In 2012, President Obama received nearly 51 percent of the national presidential vote and Mitt Romney received 47 percent — a margin of victory of almost 4 percentage points. The national vote, of course, gives equal weight to every individual’s vote. But if we give equal weight to states instead of individuals, Romney becomes the winner. The average state presidential margin favored the Republicans by nearly 2 percentage points, a six-point swing from the actual outcome. This is shown below in the first two bars:
Second, Republicans are advantaged because less populous states are more Republican than more populous states. The 25 most populous states had 258 million people based on the 2010 Census. In these states, Obama won by an average of 2.3 percentage points. But in the 25 least populous states, which contain only 50.4 million people, Romney won by an average of 6 percentage points. The second pair of bars in the graph above makes this point.
In short, both the average state and the less populous states are less Democratic and more Republican than the country overall. Thus, an institution like the Senate — which weights states rather than individuals equally — yields a benefit for the Republicans right now, regardless of the economic and political conditions this election year.