Obama has bought every election he ever has won. His weakest showing was against McCain. He outspent him by only three to one, well under his average.
He can't do that this time. Romney is now raising more money, including PACs, and, though he is still running behind, by election day there will probably be a rough parity between the campaigns' fundings, all sources and active participants considered. Since his first unsuccessful one, Obama has never had to run a race with his opponent being able to spend about as much as he. Now he does. The funding is what is putting the two candidates on a more or less equal footing this time.
All that money comes from somewhere. Were it not for a great deal of campaign contribution to Romney, both direct and indirect, Obama would now strutting cockily around confident he was buying yet another one. Instead, he's gone nastily negative in the hope of making himself appear the lesser of the evils in order to squeak by, and he isn't sure it's going to work. That's why he looks and acts so worried and angry lately.
The people willing to give are having a strong influence on this election. Don't say you won't. This one is showing that it matters.