Politicology points out that a recent ABC News/Washington Post poll has Obama barely ahead of Romney by a 48-45% margin. But, when asked about enthusiasm toward a candidate, 91% of Obama supporters are enthusaistic about voting for him while only 73% of Romney voters say the same thing.
Why the lack of enthusiasm? For all that Romney gives Obama a run for his money on handling of the economy, he has room to make up in almost every other area, particularly sincerity. Obama beats Romney on personal character, 52/38, better understands the country's financial problems, 48/40, and is more likely to stand up for what he believes in, 51/38. This means a decent amount of respondents agree with Romney, but they don't trust him. Voters like this won't vote for Obama, but they're more likely to stay home on election day. Economic arguments won't change their minds; Romney's going to have to do a better job of appealing to these voters as a trustworthy tribune of their values. In short, he's going to have to get these voters to believe in him, not just agree with him.