the Citizens United case - which gave corporations First Amendment rights

SCOTUS has generally defended the idea that corporations have first amendment rights since Grosjean v. American Press Co. in 1936 - a case where a Senator pushed for a tax designed to target papers critical of him and tax them into submission.

To quote Wikipedia on the case:

The case is often cited because it defined corporations as "persons" for purposes of analysis under the Equal Protection clause.

The Citizens United case was that a corporate entity or nonprofit distributing political messaging about a candidate is not considered a campaign contribution (even when it costs them to do so) so long as the entity in question is not attached to or coordinating with the campaign.