Except in your world the person with the most resources is the one that wins by default because they simply outspend everyone else, how is that righteous? Especially when we're not talking about individuals here, we're talking about corporations.

Besides your premise is flawed to begin with, you most certainly cannot freely rent out a billboard and put anything you want on there, or a radio ad, or a book printing. Restrictions on content already exist, and I see no reason why an exception should be made for political parties.