Despite evidence to the contrary, Jonathan Bernstein argues that the newsletters could still do Paul damage:

[T]his sounds to me a little like all of the people who were claiming a week into the anti-Newt onslaught that he was immune to attacks for whatever reasons. It's very possible that it just will take a bit of time to sink in.

But against the force of ads like the one above, there may not be enough time yet. Gingrich and Santorum have attacked Paul – but on foreign policy, not frontally on the newsletters.