Vicki Divoll, a former Senate intelligence committee general counsel and assistant general counsel for the CIA, was unambiguous about the advice she would give if CIA director she was working was asked by a president to encourage the FBI Director to shut down an investigation into associates of that president.

"I would tell my boss that the request was inappropriate and not to act on it. it's as simple as that," said Divoll. "Then we would need to discuss whether we had a statutory obligation to make a crimes report to the Justice Department and to notify the intelligence committees of Congress."

Several former intelligence officials said that they had never seen a president ask an intelligence agency chief to intervene to stop a federal investigation into the associates of that president. Such a request might be construed as an effort to obstruct justice; CIA officials are bound by law to report potential crimes to the FBI.

By publicly admitting that the Russia investigation played a role in his decision to fire Comey, and by asking intelligence officials to help him intervene, Trump has raised questions about whether he attempted to shut down a federal investigation. Even if he did, though, it’s unclear whether a sitting president could be prosecuted for obstruction, or whether impeachment would be the only means of legal accountability.

“At least since the Nixon administration and the Watergate investigation, there has been common agreement and understanding throughout the federal government that federal criminal investigations of potential executive-branch wrongdoing should proceed independently of the president and of executive-branch officials generally,” Bruce Green, a former assistant counsel on the Iran-Contra affair and a law professor at Fordham, wrote in an email. “Given the presumption of innocence, one might presume (absent compelling contrary evidence) that the president acted from lawful motives. On the other hand, if the president had ‘corrupt intent’ under the obstruction-of-justice law, he probably committed a crime and an impeachable defense.”

Part of the difficulty with prosecuting obstruction of justice is that it turns on “corrupt intent,” that is, intent to break a law. If Pompeo received a request to intercede with Comey and refused it like Coats reportedly did, he would be in the clear. On the other hand, if he received and later acted on such a request, much would depend on whether he could be said to have acted with “corrupt intent.”

Congress is supposed to be a check on political interference with intelligence gathering and law enforcement. Divoll recalled a controversy on the intelligence committee after then-Vice President Dick Cheney made what The New Republic described as “unusual visits” to Langley in 2002 that were seen as an effort to pressure Agency analysts to provide information to support the administration’s case for war in Iraq.