Donald Trump’s personal lawyer, John Dowd, has opined that it is impossible for the President of the United States to obstruct justice.  Superficially considered, and depending on what one means by “obstruction of justice,” that idea isn’t as far out as it may sound.
The operative constitutional provision is Section 3 of Article II, which in pertinent part provides that the president is to “take Care that the Laws be faithfully executed…,”  making him the top law enforcement officer of the United States. But he’s more than just your local chief of police writ large. He is, all by himself, a co-equal branch of government, along with Congress and the Supreme Court. So he is not only in charge of enforcing the laws of the United States, he is in charge of how he goes about it. He has no supervisor.
Now let’s suppose he has a discussion with the FBI director, let’s call him “James Comey,” wherein he tells Mr. Comey to be lenient in his investigation of a criminal suspect, whom we will refer to as “then-national security adviser Michael Flynn.” Your humble servant will concede mystification on how one goes about being lenient in an investigation, but we will assume that the president’s request made sense. Investigating crime is part of executing the laws, exactly the sort of thing the president is in charge of. And since he is in charge of it, he gets to decide how to do it, even if his instructions will leave native English speakers scratching their heads.
But that doesn’t mean that it is impossible for the president to commit the crime of obstruction of justice. Yes, the president may direct criminal investigations when he’s not busy insulting the North Korean dictator on Twitter, but he may not violate the criminal statutes of the United States.
For example, the president may not intimidate someone in order to keep him from telling a federal judge about a crime, a course of action whereby he could win for himself up to 20 years in prison. (18 U.S. Code, §1512(b)(3))  The president may not destroy documents so that they won’t be available in official proceedings (18 U.S. Code §1512(c)(1)), another 20 year offense. And there are many such things that a president may not do.
So when the president’s lawyer says that a president cannot commit obstruction of justice, it is easy to see what he means. But it is also clear that he didn’t think his position through very carefully.