Strictly as a matter of law, evidence that Trump ordered his lawyer to perjure himself does change things, in Posner's view. "We have a much clearer crime that is easier to prove," he said. "The president has weaker Constitutional arguments on his side than the vaguer obstruction-of-justices charges that would be based on firing [Federal Bureau of Investigation Director James] Comey, or the various statements he made that could be considered threats or promises of pardons."

But here's the rub: All that applies only in the unlikely event that special prosecutor Robert Mueller defies Department of Justice guidelines stating that a sitting president cannot be indicted. Otherwise, bringing Trump to justice falls to Congress, which will, in turn, be guided by public opinion.

Posner played out that scenario as part of a wide-ranging interview.

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