Danny Cevallos ( @CevallosLaw ) is a CNN Legal Analyst and an attorney practicing in the areas of personal injury, wrongful conviction, and criminal defense in New York, Pennsylvania, and the US Virgin Islands. The opinions expressed in this commentary are his.

(CNN) The release of James Comey's opening statement has already prompted some legal analysts to argue that President Donald Trump obstructed justice. But even if everything James Comey says is true, is an obstruction case open and shut?

President Trump could have avoided this mess completely. He didn't need to ask Comey to "let go" a criminal investigation of Flynn. He could have just pardoned Flynn himself, bypassing Comey completely. The President can pardon any federal offender, even in advance of prosecution

Is it possible then the President could be accused of criminal or impeachable conduct for asking someone else to do something he could have done himself, with impunity? That would be a constitutional paradox, for sure.

A Flynn pardon would have undoubtedly set off a political firestorm. So would an order from the president to the Department of Justice, which reports to him, to call off the investigation of Flynn.

If Comey's opening statement is accepted as credible, and for those who do not believe the President has unfettered authority to call off a DOJ investigation, then the President may have crossed legal, moral and even impeachment lines.

Clearly one difference between what Trump allegedly said and a lawful use of a president's authority is that, according to Comey, the president made his wishes known to him in secret. And those statements which Comey relates draw a picture of a chief executive who doesn't appear to understand the usual legal and ethical boundaries that presidents are expected to observe.

Assume for the moment -- and it's a "bigly" assumption -- that the act of telling Comey to "let [the Flynn investigation] go" meets the elements of one of the many obstruction of justice statutes : obstruction of judicial proceedings, witness tampering or retaliation, or obstruction of congressional or administrative proceedings.

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Would the President's ignorance of the complex obstruction laws be a defense? Could Trump, a president with no prior government experience, argue in his defense that he didn't think what he did violated any law?

It's all up to Congress to decide.

The general, well-known rule is that ignorance of the law is not a defense to a crime. So ingrained is this maxim that most lay people can recite it. At the same time, it's a complete legal fiction. Everyone is ignorant of the law to some degree—attorneys and citizens alike. The rule therefore prevents a situation where people get acquitted for murder because of the "I didn't know shooting people was illegal" defense.

Yet Congress can always override this default rule. It can do so by simply writing a requirement into a piece of legislation that a defendant act willfully or with specific intent to violate the law. For example, in the overwhelmingly complex area of tax law, Congress softened the presumption that ignorance of the law is no excuse by making specific intent to violate the law a required element of certain federal criminal tax offenses. In these tax cases, the prosecution must prove the defendant knew his legal duty, and voluntarily and intentionally violated it.

In the case of obstructing justice, it appears that knowledge of one's violation is not one of the required elements , at least according to the federal appellate courts.

In the witness tampering statute, for example, there is no requirement that the defendant act willfully, or that he knows his conduct is unlawful. A defendant could therefore obstruct justice with an improper purpose even if he didn't know the actions were illegal. As long as a defendant intended to subvert, undermine or impede the fact-finding ability of an official proceeding, that's enough to convict him.

And that's just obstruction of justice. A creative prosecutorial mind could come up with any number of theoretical criminal acts if Comey's must-see testimony yields some damning evidence on Thursday. For example, there's the rarely used misprision statute -- where one doesn't take part in a crime, but knows of it, fails to inform law enforcement and takes affirmative steps to conceal the crime of another. Hypothetically, if Flynn committed a crime that was known to the President, one could argue that steering the FBI away from the "good guy" Flynn was a form of misprision.

Even if ignorance of the law is a defense to a particular federal crime, that doesn't mean it will be a defense to impeachment. An impeachable offense need not even be a criminal offense, so there's no reason why ignorance of the law would be a defense to impeachment. And while the President can probably even pardon himself from criminal prosecution, he absolutely cannot pardon himself from impeachment.

If Congress is incensed enough at a perceived abuse of power, Congress alone can impeach the President. Whether or not ignorance of the law is available as defense to impeachment is not up to the courts or the language of federal criminal statutes. That, too, is completely up to Congress.