Individuals, Companies, and Corporations:
No person (including corporations) other than a candidate’s official campaign organization, may pay for political advertising promoting or denouncing any candidate.
Note this does not preclude public statements endorsing or denouncing a candidate, including newspaper and TV editorials. But it must be printed/aired just one time during a campaign. “On demand” reading/viewing of the editorial is allowed, but no person other than the campaign itself is “paying” to promote or advertise said editorial/endorsement.
Political parties:
Political parties are to be funded by donations from US citizens and permanent residents who are constituents of the area represented by that party. Notice that excludes corporations and foreigners. Parties may not pay for activities that promote or oppose any specific candidate, but they may pay for advertising promoting their party “platform”, views on issues, etc. They may also pay for their national presidential nominating convention, and state parties may pay for conventions for their nomination of state governors and other statewide office holders. National parties may not fund state parties nor vice-verse, but they may help promote or hold fund-raising events for each other. This is to ensure that state parties are nearly free or outside influence from other states. Parties may of course maintain, publish, and distribute lists of their candidates and elected members.
PACs:
Political Action Committees are permitted but may only promote or oppose specific issues/laws, they may not promote or oppose any candidate or political affiliation (e.g. Political Party). They can accept donations from US corporations and any US citizen or permanent resident. This is how corporations can express their political views, by forming and/or contributing to PACs that promote or oppose issues of importance to that corporation. Note: this could include churches and other not-for-profit organizations. This prohibition includes purely “internal” communications that promote or oppose specific candidates or parties. E.g. preachers promoting a candidate from the pulpit, memos/email from corporate executives about candidates or parties, etc. PACs may have internal communications about which candidates support or oppose (including undecided or unknown) the issues the PAC supports or opposes, as such is essential to their mission, and they may publish lists of candidates CONFIRMED to PUBLICLY support or oppose each issue/law, so long as they have performed due diligence in verifying the accuracy of those lists before publishing. Candidates who haven’t publicly stated a position on the issue/law may only be listed as “position unspecified”