Citizens United v. Federal Election Commission was a landmark U.S. Supreme Court case in which the Court held that the First Amendment prohibited the government from restricting independent political expenditures by corporations and unions. More: https://www.amazon.com/gp/search?ie=UTF8&tag=tra0c7-20&linkCode=ur2&linkId=b154493591316c088236baa99d4930a0&camp=1789&creative=9325&index=books&keywords=citizens%20united
The nonprofit group Citizens United wanted to air a film critical of Hillary Clinton and to advertise the film during television broadcasts in apparent violation of the 2002 Bipartisan Campaign Reform Act (commonly known as the McCain--Feingold Act or "BCRA"). In a 5--4 decision, the Court held that portions of BCRA §203 violated the First Amendment.
The decision reached the Supreme Court on appeal from a July 2008 decision by the United States District Court for the District of Columbia. Section 203 of BCRA defined an "electioneering communication" as a broadcast, cable, or satellite communication that mentioned a candidate within 60 days of a general election or 30 days of a primary, and prohibited such expenditures by corporations and unions. The lower court held that §203 of BCRA applied and prohibited Citizens United from advertising the film Hillary: The Movie in broadcasts or paying to have it shown on television within 30 days of the 2008 Democratic primaries. The Supreme Court reversed, striking down those provisions of BCRA that prohibited corporations (including nonprofit corporations) and unions from spending on "electioneering communications".
The decision overruled Austin v. Michigan Chamber of Commerce (1990) and partially overruled McConnell v. Federal Election Commission (2003). The Court, however, upheld requirements for public disclosure by sponsors of advertisements (BCRA §201 and §311). The case did not involve the federal ban on direct contributions from corporations or unions to candidate campaigns or political parties, which remain illegal in races for federal office.