There is a finite number of spots available.
Granted that number may be rather large, but it is still finite.
After a while, the messaging in the media reaches a point of diminishing returns, an overload of messaging that will turn some voters off and may actually cause some to move away from a candidate that they view as too shallow or too negative.
At the end of the day, it is going to be the people in neighborhoods convincing their neighbors to vote for their candidate
I believe that the Obama team has the best message and the better ground forces in the swing states. Especially if you compare th Obama campaign with the McCain campaign.
Remember this as well, McCain was liked by the voters. He has a special place on the national scene and is viewed as a "moderate" when it comes to Republicans.
Romney is the rich know it all who every body would like to be far away from. He is far from being likable and runs th risk of coming off as a bully.
So, I am pretty cautiously confident that we will prevail. Now if only we can take better control of the Senate and continue to show real leadership in that august chamber and take down some of th loud mouth newbies in the House we might be able start turning the economy around and let Obama get on with his agenda...