This midterm election will be the most expensive in American history due to the landmark Supreme Court decision Citizens United vs. FEC which essentially equates campaign spending with political speech, protecting it under the 1st amendment.
Here are extracts from Justice Stevens dissenting opinion:
In the context of election to public office, the distinction between corporate and human speakers is significant. Although they make enormous contributions to our society, corporations are not actually members of it. They cannot vote or run for office. Because they may be managed and controlled by nonresidents, their interests may conflict in fundamental respects with the interests of eligible voters. The financial resources, legal structure, and instrumental orientation of corporations raise legitimate concerns about their role in the electoral process. Our lawmakers have a compelling constitutional basis, if not also a democratic duty, to take measures designed to guard against the potentially deleterious effects of corporate spending in local and national races