Peter Suderman explains Paul's success:
[O]ne thing you can say about Paul is that he is not offering anything that could be described as conventional Republicanism; his campaign is built on opposition to defense spending and overseas adventurism, a critique of the Federal Reserve, and a return to constitutionally limited government. Compare this to the shrugging acceptance with which Romney’s vanilla campaign and laundry list of GOP priorities have been greeted; Paul, in contrast, has managed to generate tremendous, unusual enthusiasm. Indeed, he’s the only candidate in the race who has been able to sustain and build such enthusiasm over time. Who knew? The most effective anti-Romney turns out to be someone who is genuinely not like Mitt Romney.
Massie builds on this thought:
[Paul] is, whatever his shortcomings, the purest rejection of Bushian conservatism available to voters this time around. Moreover, there's a touch of Howard Beale about Paul and the Paulites. They're mad as hell and unwilling to take it any longer.
The Dish 