[T]he case against Mr. Trump involves three key events. First, James Comey said that when he was the F.B.I. director, the president told him in a Valentine's Day chat, "I hope you can see your way clear to letting this go, to letting Flynn go." Two Federal Courts of Appeals have held that similar "I hope" statements can -- depending on the context -- support charges of obstruction.

Second, President Trump reportedly asked the director of national intelligence, Daniel Coats, in a private meeting in late March if Coats could get the F.B.I. to back off its Flynn probe. President Nixon's attempt to use the C.I.A. to shut down the Watergate investigation was one of the reasons the House Judiciary Committee voted for articles of impeachment on obstruction charges.

Last, President Trump fired Mr. Comey on May 9 and then said on television that the firing was related to the Russia inquiry -- a signal to Comey's replacement, Acting Director Andrew McCabe, that he should roll back the investigation if he wanted to stay on as F.B.I. chief.

Even if none of those specific incidents would qualify as obstruction on its own, federal courts have said that an entire course of conduct can constitute obstruction. And whether Mr. Trump succeeded in his efforts is legally irrelevant, because federal law criminalizes attempted obstruction as well as successful obstruction. Nor does it matter whether there was an actual underlying crime.