3. Is there a ban on corporate donations to political parties?

Paragraph 441b (a) It is unlawful for any national bank, or any corporation organized by authority of any law of Congress, to make a contribution or expenditure in connection with any election to any political office, or in connection with any primary election or political convention or caucus held to select candidates for any political office, or for any corporation whatever, or any labor organization, to make a contribution or expenditure in connection with any election at which presidential and vice presidential electors or a Senator or Representative in, or a Delegate or Resident Commissioner to, Congress are to be voted for, or in connection with any primary election or political convention or caucus held to select candidates for any of the foregoing offices, or for any candidate, political committee, or other person knowingly to accept or receive any contribution prohibited by this section, or any officer or any director of any corporation or any national bank or any officer of any labor organization to consent to any contribution or expenditure by the corporation, national bank, or labor organization, as the case may be, prohibited by this section.

United States Code, Title 2, The Congress. 

Corporations and unions cannot make direct contributions to parties and federal candidates, but they can make contributions through a PAC, subject to limitations.