Tobin believes that Paul's strong showing in IA and NH is a result of those contests being "open." That is, being contests where Democrats or independents may choose to show up and vote in the GOP primary. He notes that most states remaining are "closed," only registered Republicans may vote in their primaries.
It is difficult to imagine many serious Republicans voting for Paul, most of his votes must come from elsewhere. Tobin may have identified the source; we will learn the answer in the next couple of months.