Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" (Times Books) and of the new book "Governing America" (Princeton University Press).

Princeton, New Jersey (CNN) -- President Barack Obama's re-election campaign got off to a rocky start with his attacks on Mitt Romney's record at Bain Capital, the private investment fund....

But the first few weeks of attacks have backfired. Newark Mayor Cory Booker, a rising star of the Democratic Party, said that he didn't agree with the strategy, calling the ads "nauseating" and defending venture capital. Massachusetts Gov. Deval Patrick made similar statements about private industry, fueling the impression Obama's surrogates were not on the same page as him....

In short, the attacks don't seem to be working, at least thus far. Why? The first reason is that these kinds of ads are tricky for Obama to pull off since they open him up to attack as much as they allow him to inflict wounds on Romney....