Obama thinks there might be a link between autism and immunizations involving thimerosal. The campaign is saying that he doesn't necessarily believe that; he was just acknowledging that others do. But he did say that the science is inconclusive on the subject, and that's bullshit.
I don't know why he feels compelled to do this. One possibility is that he believes it. Another is that he sees a political percentage in cozying up to the autism-thimerosal-link folks. I don't know how many of them there are, but they're surely more passionate than the people who are saying that there is no link, and they may even vote on the issue.
Regardless, it's totally inappropriate and irresponsible for him to be saying this stuff. He's bright and should know better. Beyond that, he's a respected figure with a large and devoted following, and parents of young kids might take him seriously. And that could lead to more of this.
I'm hoping losing Pennsylvania will be penance enough.