Charlie Savage for the NYT:

What is a big challenge to prosecuting Mr. Trump?

Proving Mr. Trump’s state of mind — specifically, that he had the requisite criminal intent.

The obstruction statute, for example, says that for the defendant’s action impeding an official proceeding to be a crime, he had to act “corruptly.” But what that means is not detailed in the statute, and the Supreme Court has not definitively offered an answer, raising risks and complications for prosecutors evaluating a potential case.

One possibility, said Laurie L. Levenson, a criminal law professor at Loyola Law School in Los Angeles, is that prosecutors would have to prove that Mr. Trump knew for sure that Mr. Pence had no lawful basis to do what he was asking. Another possibility is that prosecutors would need to prove only that Mr. Trump had at least some reason to believe that his conduct might be unlawful and proceeded anyway, she said.

Why is proving Mr. Trump’s mind-set tricky?

Because even though senior government officials were telling him there was no factual or legal basis for Mr. Pence to unilaterally reject some states’ electoral votes or otherwise slow down the certification, Mr. Eastman told Mr. Trump that he interpreted the law as giving Mr. Pence legitimate authority to take such a step.

Julie O’Sullivan, a Georgetown University criminal law professor, said in any criminal trial, it would ultimately be up to the jury to decide what Mr. Trump truly believed. Unless evidence emerges that he told someone at the time that he knew what he was saying was false, she said, that will be a challenge.

“The problem with Trump is defining his state of mind when it is so changeable,” she said. “He believes whatever he wants to think and it doesn’t necessarily have to be grounded in reality. That’s a tough argument to a jury, to say he knew any particular thing.”