Paul has also run advertisements attacking Romney’s key rivals at critical times. He ran hundreds of thousands of dollars in brutally negative ads attacking Gingrich in Iowa. Paul now is using his scarce funds on a television ad attacking Rick Santorum in Michigan, a key state where Paul is a non-factor. Paul is effectively acting as Romney’s on-stage surrogate during the debates. The key question is: what is Paul getting out of it?
Brian Doherty points out that Think Progress doesn't define what qualifies as an attack:
I certainly think these examples of Paul specifically criticizing specific things Romney said–on Afghanistan and the NDAA and military "spending cuts"–might qualify as "attacks" but again I have no idea what ThinkProgress's criteria are, so there's not much more I can say about that. Specific attacks on his opponents out of his own mouth, unless he's asked or is directly responding to a dumb thing his opponents just said, isn't a dominant part of Paul's style regardless.