The Federal Election Commission has a three-pronged test for proving coordination: Did an outside group pay for ads, phone calls or mailers? Did these materials tell people to vote for or against a candidate, or praise or criticize a candidate in the weeks before an election? Finally, did the candidate, or a representative, agree to the expenditure?
The FEC’s three-prong test obviously has one tyne too many … the last one is virtually unprovable.
Wobbly Warrior's Blog 