Eric Posner, a law professor at the University of Chicago, also said that Mr Comey's oral testimony had changed the legal picture.
"Comey may not have added specific pieces of information, but the way he presented himself and consistency of his account, and the coherency of the account, all of these things are important," he said.
"In his oral testimony, he made it clearer that he believed Trump was trying to obstruct justice. He said that when Trump used the word hope, he understood that to be a command of some sort, and that strengthens the case for obstruction.
"It was a borderline case before, now it is somewhat stronger."
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