In the Citizens United v. FEC case, what amendment was referenced?

The case of Citizens United v. FEC primarily referenced the First Amendment, which guarantees the rights to freedom of speech and expression. The Supreme Court's decision emphasized that political spending is a form of protected speech under the First Amendment. In this landmark case, the Court ruled that corporations and unions could spend unlimited amounts of money on political campaigns, arguing that such expenditures promote free expression and increase public discourse in the political sphere. This interpretation of the First Amendment underscores the idea that restrictions on independent political spending by these entities could infringe upon the fundamental right to free speech.