The United States is rare in the world for having a campaign finance system that is centered on candidates, rather than parties. In the U.S., for the most part, candidates must take primary responsibility for raising campaign funds. In most democratic countries, parties take on that burden for their nominees. Political Science Professor Raymond J. LaRaja has this article in The Forum. It suggests that U.S. government would work better if campaign finance was more oriented through political parties.
The U.S. Supreme Court is responsible for the current state of federal campaign finance law. The Court has struck down many campaign finance laws, but has left intact the inhibitions on political parties in the McCain-Feingold Law. Political parties have less ability to make independent expenditures than any other type of group. Thanks to The Mischiefs of Faction for the link.