How much does control of the Senate matter? Not much, David Friedman notes:
Nothing can get passed if either party is solidly opposed to it.
Which brings me back to my theory of why people vote. It isn’t to change the political outcome, since any reasonable person know that, in a large population polity, his vote has virtually no chance of doing that. It’s for the same reason people go to football games—to cheer for their side.
In order to have a game you need some definition of winning and losing. In order for it to be interesting, the definition has to leave the outcome in doubt. If winning the midterm elections was defined by whether or not the Republicans retained their majority in the House or by whether they gained enough seats in both houses to override a presidential veto, it would have been a very boring contest, since the answer to both questions was known long in advance.
Viewing it as a contest over who ended up in control of the Senate, on the other hand, made it a game worth watching.