Citizens United v. Federal Election Commission 558 U.S. 310 (2010) is a landmark U.S. constitutional law, campaign finance, and corporate law case dealing with regulation of political campaign spending by organizations - wikipedia 
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The United States Supreme Court held (5–4) on January 21, 2010 that freedom of speech prohibits the government from restricting independent expenditures for communications by nonprofit corporations, for-profit corporations, labor unions, and other associations.
In the case, the conservative non-profit organization Citizens United wanted to air a film critical of Hillary Clinton and to advertise the film during television broadcasts shortly before the 2008 Democratic primary election in which Clinton was running for U.S. President.
This would violate a federal statute prohibiting certain electioneering communications near an election. The court found the provisions of the law that prohibited corporations and unions from making such electioneering communications to conflict with the U.S. Constitution.
However, the court upheld requirements for public disclosure by sponsors of advertisements. The case did not affect the federal ban on direct contributions from corporations or unions to candidate campaigns or political parties.
The decision was highly controversial when announced and remains a subject of much discussion.