In this instance there’s at least prima facie evidence that would tend to support inferences of obstruction. According to the memo, after all, a conversation took place in which the President asked the FBI director to “see your way clear to letting this go, to letting Flynn go,” and, in the [New York] Times’s words, “told Mr. Comey that Mr. Flynn had done nothing wrong.” So assuming the memo is accurate, there’s at least an act that a reasonable person would understand as seeking to influence the investigation. As the Times story notes, “[t]he existence of Mr. Trump’s request is the clearest evidence that the president has tried to directly influence the Justice Department and F.B.I. investigation into links between Mr. Trump’s associated and Russia.” While in and of itself, the request could be understood as just a plea for mercy, which is not obviously obstructive, the fact that it comes from a superior with the power to remove the investigator—alongside the fact that Trump then did fire Comey—makes obstruction a plausible reading of the apparent facts.

There are other elements here that also make a case more plausible. For one thing, there’s a contemporaneous memo. There’s also a witness: Comey himself, who could presumably testify as to the circumstances of the meeting. To the extent anyone claimed his story was a subsequent fabrication, the memo could be used as evidence to rebut that claim.