On January 21, 2010, the Supreme Court of the United States rules 5-4 that corporations and unions have the right to spend unlimited amounts of money on political speech, such as advertisements, as long as they do so "independently." The decision, known as Citizens United v. the Federal Election Commission, cited the logic that more speech is better than less, and it is only acceptable to limit political speech (of which spending is a form) when it causes corruption. As long as spending isn't coordinated with a candidate's campaign, the Court held, it does not lead to corruption, so its limitation is unjustified. 80% of Americans oppose this decision.
Source: ABC News

"Citizens United decimated what remained of campaign-finance reform, but the damage has been long in the making." For more information on the impact of Citizens United and corporate personhood, read this article in The American Prospect by Garrett Epps.