Which ruling confirmed that corporations could donate unlimited amounts to nonprofit arms of PACs?

The ruling that confirmed corporations could donate unlimited amounts to the nonprofit arms of political action committees (PACs) is Citizen's United v. FEC. This landmark decision, issued by the Supreme Court in 2010, held that the First Amendment protects the right of corporations and unions to spend unlimited amounts of money on political advocacy, as long as the spending is independent of any candidate's campaign.

The Court reasoned that political spending is a form of protected speech, and therefore, limiting the amount that corporations could contribute would be a violation of their free speech rights. The decision allowed for a significant increase in the amount of money flowing into the political system through corporate donations and independent expenditures, fundamentally altering the landscape of campaign finance.

Other cases like Baker v. Carr and Bush v. Gore do not pertain to the issue of corporate donations and campaign finance regulations. McConnell v. FEC involved an earlier challenge to campaign finance laws but did not reach the expansive impact that Citizen's United did regarding corporate spending.