Let us presume that Common Cause and other reformers are clever enough to use monetary incentives to severely restrict the amount of money that campaigns and parties can spend. Political parties are extremely important in insuring a competitive political process and the exchange of ideas that nurtures political discourse. Parties provide support and advice to novice political candidates, unavailable elsewhere and are often the only support for candidates running against powerful and well-known incumbents. Limiting party spending would endanger these activities. In addition, severe limits on campaign spending tilt the political process in favor of incumbents.
Incumbents have name recognition and the public treasury with which to buy attention and gratitude.
Less well-known outsiders must often outspend incumbents if they wish to be competitive. As long as the governments spend a large fraction of the national income and so long as regulations and subsidies may have a large impact of businesses, labor and other groups, these forces will legitimately want to make the case for their positions. If individuals and groups are not permitted to give to campaigns or parties directly, they will seek alternative means to advocate their causes. There would be a strong incentive to funnel money illegally. However, even if all illegal funding was successfully prevented, there is nothing to stop independent expenditures by all sorts of groups.