The sad fact is limiting spending during campaigns is limiting free speech and it is hard to make rules limiting free speech during an election stick. The best you can do is argue why you think you should be able to limit someone else's free speech and that is never easy. Both parties could agree to a set of gentleman's rules and the press would hopefully beat the crap out of anyone who violated them, even if they are a democrat. The problem is both sides always try insist that the rules should fixed to favor them so they never agree. That is why so many republicans were pissed at McCain over McCain-Feingold, they thought it hampered republicans more than dems.

Even a cap is hard to work since the incumbent doesn't need to spend money on a primary. I have yet to see a really good workable way to control the beast.