A great graphic put together by the New York Times.

Looking at the chart, Paul is apparently following the advice of Tom Roeser, who advised him to stick to radio:

Running the campaign on vol­unteers saves money for paid communications. By which I mean radio. A decade ago a guest at my political science class at De Paul University was Michael Deaver ( who died recently). Everyone be­lieves Ronald Reagan was the most popular governor California ever had. Not so. He won his sec­ond term by only 52% in 1970. But he still wanted to run for pres­ident.

He turned to Deaver, who under­stood the governor was a conser­vative ideologue ( as Deaver de­cidedly was not). Radio, he rea­soned, was for the philosophical­ly committed, the people Reagan had to appeal to. So he put Reagan on the radio across the country — radio exclusively.

Each radio message of only a few minutes in length had him deliv­er small bits of conservative phi­losophy in bite-sized morsels. At the end he would say, “ This is Ronald Reagan. Thanks for listen­ing.”

While Reagan was known na­tionally from his films, his ideas — aside from California — were not. Radio got ex-radio announc­er Reagan across to the country.

Romney, on the other hand, has ran 17,000+ TV commercials to put him at a whopping 15% in the national polls.