In this case, the Supreme Court ruled that any laws that try to restrict the political spending of corporations and unions is a violation of the First Amendment’s right of free speech.

In a dissenting opinion, Justice John Paul Stevens argued that the court's ruling represented "a rejection of the common sense of the American people, who have recognized a need to prevent corporations from undermining self government".

[3] The decision remains highly controversial, generating much public discussion and receiving strong support or opposition from various politicians, commentators, and advocacy groups.

The FEC, however, held that showing Celsius 41.11 and advertisements for it would violate the Federal Election Campaign Act, because Citizens United was not a bona fide commercial film maker.

On January 15, 2008, the court denied the Citizens United motion for a preliminary injunction, finding that the suit had little chance of success because the movie had no reasonable interpretation other than as an appeal to vote against Hillary Clinton.

[21] According to a 2012 retrospective article in The New Yorker by Jeffrey Toobin, the court planned to rule on the narrow question that had originally been presented: Can Citizens United show the film?

"[28] Kennedy also noted that because the First Amendment does not distinguish between media and other corporations, the BCRA restrictions improperly allowed Congress to suppress political speech in newspapers, books, television, and blogs.

"[27] The court overturned the 1990 precedent Austin v. Michigan Chamber of Commerce, which had held that a state law that prohibited corporations from using money to support or oppose candidates in elections did not violate the Constitution.

[2] The court's opinion relied heavily on Buckley and First National Bank of Boston v. Bellotti, in which it struck down a broad prohibition of independent expenditures by corporations in ballot initiatives and referendums.

[29][30]Chief Justice John Roberts wrote a separate concurring opinion "to address the important principles of judicial restraint and stare decisis implicated in this case".

Had prior courts never gone against precedent, for example, "segregation would be legal, minimum wage laws would be unconstitutional, and the Government could wiretap ordinary criminal suspects without first obtaining warrants".

To protect the anonymity of contributors to organizations exercising free speech, Thomas would have struck down those reporting requirements, rather than allowing them to be challenged only on a case-by-case basis.

[27] On the matter of undue corporate influence on elections and spending on behalf of chosen candidates, Stevens cited First National Bank of Boston v. Bellotti and argued that the high court had "never suggested that such quid pro quo debts must take the form of outright vote buying or bribes".

[43] While the long-term legacy of this case remains to be seen, an early study by one political scientist has concluded that Citizens United worked in favor of the electoral success of Republican candidates.

[52] Campaign finance attorney Cleta Mitchell, who had filed an amicus curiae brief on behalf of two advocacy organizations supporting Citizens United, wrote that "The Supreme Court has correctly eliminated a constitutionally flawed system that allowed media corporations... to freely disseminate their opinions about candidates using corporate treasury funds, while denying that constitutional privilege to Susie's Flower Shop Inc. ...

"[54] John Samples and Ilya Shapiro of the Cato Institute disagreed with the idea "that corporations had so much money that their spending would create vast inequalities in speech that would undermine democracy".

It is a lot easier to legislate against unions, gun owners, 'fat cat' bankers, health insurance companies and any other industry or 'special interest' group when they can't talk back.

"[58] President Barack Obama stated that the decision "gives the special interests and their lobbyists even more power in Washington—while undermining the influence of average Americans who make small contributions to support their preferred candidates".

Presented with a relatively narrow legal issue, the Supreme Court chose to roll back laws that have limited the role of corporate money in federal elections since Teddy Roosevelt was president.

"[68][69] When discussing the ruling and related developments, former President Jimmy Carter called the United States "an oligarchy with unlimited political bribery" in an interview with Thom Hartmann.

[70] Retired Supreme Court Justice Sandra Day O'Connor, whose opinions had changed from dissenting in Austin v. Michigan State Chamber of Commerce to co-authoring (with Stevens) the majority opinion in McConnell v. FEC twelve years later, criticized the decision only obliquely, but warned, "In invalidating some of the existing checks on campaign spending, the majority in Citizens United has signaled that the problem of campaign contributions in judicial elections might get considerably worse and quite soon.

"[71] Constitutional law scholar Laurence H. Tribe wrote that "talking about a business corporation as merely another way that individuals might choose to organize their association with one another to pursue their common expressive aims is worse than unrealistic; it obscures the very real injustice and distortion entailed in the phenomenon of some people using other people's money to support candidates they have made no decision to support, or to oppose candidates they have made no decision to oppose.

A lobbyist can now tell any elected official: if you vote wrong, my company, labor union or interest group will spend unlimited sums explicitly advertising against your re-election.

[80][81] Separate polls commissioned by various conservative organizations, including Citizens United and the Institute for Free Speech, using different wording, found support for the decision.

[84] In February 2010, shortly after the Supreme Court ruling, Senator Charles E. Schumer and Representative Chris Van Hollen outlined legislation aimed at undoing the decision.

[75] Representative Donna Edwards and Maryland Democratic State Senator Jamie Raskin, have circulated petitions to reverse the decision by means of constitutional amendment.

[101] Critics predicted that the Citizens United ruling would "bring about a new era of corporate influence in politics", allowing companies to "buy elections" to promote their financial interests.

[105] According to a 2021 study, the ruling weakened political parties while strengthening single-issue advocacy groups and super PACs funded by billionaires with pet issues.

A number of partisan organizations such Crossroads Grassroots Policy Strategies and 21st Century Colorado have since registered as tax-exempt 501(c)(4) groups and have engaged in substantial political spending.

[112] While the long-term legacy of the ruling remains to be seen, studies by political scientists have concluded that Citizens United worked in favor of the electoral success of Republican candidates.