Campaigns already have strong limits. They get around it by giving money to PACs, which can spend money opposing other candidates or talking about issues in particular.

Should there be limits on spending on messaging opposing a certain candidate? Or supporting / opposing a specific policy? It's going to be pretty tough to draw a line there that doesn't sound a lot like solidifying and entrenching the powers that already exist. And it usually includes an ugly way of determining what speech is "political" and what is not.