The letter warned that if the outlet and its reporters “continue their reckless campaign of defamation, President Trump will evaluate all legal remedies.”

The only legal remedy they’re suggesting here is a defamation/libel suit. If Trump is asserting that ProPublica’s reporting on these oh so coincidental payments is false, then he will have to prove its falsehood. Furthermore, he will have to prove that ProPublica acted with actual malice.

• “Generally, to prove defamation, you must show that a false statement was made, about you, to third parties, and which caused you damage.”
• “To win a libel suit, a public figure must prove the publisher of the false statements acted with actual malice. Actual malice means that the publisher knew that the statements were false or acted with reckless disregard for whether they were true or false. This is much harder to prove than negligence.”

You’re out of your element, Donny.