I think Democrats underestimate Paul, considering the polls.
Not to mention that most of Paul's softness is on the right - all it would take is a come to Jesus moment in the GOP to make this a very ugly scenario for Obama.
I've said this many, many times before. The reason he isn't generally leading Obama compared to Romney is the "sour grapes" effect, similar to the mythical Hillary voters that would flip were Obama elected (And they never ended up sticking to that). Similarly, there are a group of Republicans that would rather Paul not win, and thus say "undecided" when the poll comes up, which appears to lower the bonus Paul gets with independents. However, I'll eat my own socks if a good 75% of those Republicans don't come out and vote for him on election day since most of them are the sort of "Party Line" Republicans that would vote for Mussolini so long as there was an R beside his name.