As if they don't now?!?!?!
McCain ran on a shoestring budget during some of the 2008 campaign when everyone had written him off. He didn't have much money to spend and he won anyway.
All I am saying is a national primary day would virtually guarantee the richest candidate(s) swamps the key media markets and wins the nomination every time. Candidates would never have an opportunity to emerge and get stronger as the primary season progresses. It's just not a good idea at all.