@Blickered,
Politico wrote:The courts have recognized repeatedly that a government official's clear legal authority to take some action does not immunize that official from prosecution for crimes relating to the exercise of that authority. To take just a few examples, in U.S. v. Smith, several members of the Los Angeles Sheriff's department were convicted on obstruction charges for relocating and restricting access to a prisoner--conduct that would have been legal but for its purposeful interference with an FBI investigation into civil rights violations at Los Angeles County jails. In U.S. v. Baca, the court explained that "[a] local [police] officer may not use [his] authority to engage in what ordinarily might be normal law enforcement practices, such as interviewing witnesses, attempting to interview witnesses or moving inmates, for the purpose of obstructing justice." And in U.S. v. Mitchell, the U.S. Court of Appeals for the 4th Circuit upheld the conviction of two brothers who accepted a payment of $50,000 to convince their uncle--a congressman--to stop a congressional investigation into a company's eligibility for a government program.
Those cites are all cases where someone tried to impede an investigation. No one is arguing that impeding an investigation isn't obstruction. For example, it was obstruction when Bill Clinton sent Betty Curie out to hide his gifts to Lewinsky before the Starr investigation could find them.
What Trump did, however, is decide that an investigation should not go forward. It is a rather extreme leap to presume that it is obstruction for an official who is directly in charge of an investigation to decide that the investigation should not be pursued.
In addition, those cites are all cases of officials whose authority is not backed with the full might of the Constitution. Congress does not have the authority to pass a law limiting the President's control over the executive branch.
As an aside, note that the Democrats established a precedent that it is OK for presidents to commit obstruction when they let Bill Clinton off the hook. They'd have no business complaining even if this really had been obstruction.