Republican presidential candidate Donald Trump is warning that electing rival Hillary Clinton could mire the country “in a constitutional crisis that we cannot afford.”

It is not clear what Trump means by a “constitutional crisis” but he suggested Monday to supporters in Michigan that Clinton could face a lengthy criminal investigation and possible a criminal trial as a sitting president as the FBI continues to examine her email practices.

There is no agreed upon definition of the term, according to Stanford Law Professor Pamela Karlan, who specializes in constitutional law.

“Constitutional crisis is one of those phrases like ‘judicial activism’ that has a gazillion different meanings to different people,” she said. “I think it’s something where the Constitution literally can’t operate or there’s nothing in the Constitution to provide for how things will operate.”