Which case established that corporations have speech rights equivalent to those of individuals in political campaigning?

The case that established that corporations have speech rights equivalent to those of individuals in political campaigning is indeed Citizen's United v. FEC. This landmark Supreme Court decision, rendered in 2010, fundamentally changed the landscape of campaign finance in the United States. The Court ruled that restrictions on independent expenditures for political speech by corporations and unions violate the First Amendment, which protects free speech.

The significance of this ruling lies in its assertion that corporations can spend unlimited amounts of money to advocate for or against political candidates, just as individuals can. This decision was grounded in the belief that political speech should not be suppressed, regardless of the speaker's identity, thereby equating the speech rights of corporations with those of individuals.

In contrast, other cases mentioned in the options addressed different aspects of electoral politics and governance. Baker v. Carr focused on the issue of legislative apportionment and the principle of "one person, one vote." McConnell v. FEC dealt with the constitutionality of certain campaign finance laws but did not extend speech rights to corporations as broadly as Citizen's United. Bush v. Gore was primarily a case about the presidential election's voting process and the recount in Florida, lacking a focus on corporate speech rights. Thus, Citizen's United v.