"""

> Although laws vary by state, in the United States a defamation action typically requires that a plaintiff claiming defamation prove that the defendant:

> 1. made a false and defamatory statement concerning the plaintiff;

> 2. shared the statement with a third party (that is, somebody other than the person defamed by the statement);

> 3. if the defamatory matter is of public concern, acted in a manner which amounted at least to negligence on the part of the defendant; and

> 4. caused damages to the plaintiff.

"""

https://en.wikipedia.org/wiki/Defamation#Civil_defamation

Arguably all of these points are met by a "too big to jail" billboard facing the public. Though given the nature of a political candidate and free speech, I think this would be protected. Particularly because "Defenses to defamation that may defeat a lawsuit, including possible dismissal before trial, include the statement being one of opinion rather than fact or being 'fair comment and criticism'."

That said, even if it doesn't meet the technical level to convict I think it's a reasonable thing to refer to as defamation colloquially and I suspect you know that too.

For clarity on the specifics of a 'defamatory statement':

> A defamatory statement is a false statement of fact that exposes a person to hatred, ridicule, or contempt, causes him to be shunned, or injures him in his business or trade.