But when President Trump accuses Obama of an act that would have been impeachable and possibly criminal, that’s something much more serious than libel. If it isn’t true or provable, it’s misconduct by the highest official of the executive branch.
How is such misconduct by an official to be addressed? There’s a common-law tort of malicious prosecution, but that probably doesn’t apply when the government official has no intention to prosecute.
The answer is that the constitutional remedy for presidential misconduct is impeachment.