Beyond those bottom-line conclusions, Mr. Mueller’s full report has yet to be released, and it remained unclear if it ever would be. House Democrats have demanded that it be sent to them by next Tuesday, but the Justice Department outlined a longer schedule, saying that it will have its own summary ready to send to lawmakers within weeks, though not months.

So far, there are no plans to give a copy of the report to the White House in advance of any public release, according to a Justice Department official.

At the heart of the obstruction question is when a president’s use of his otherwise legitimate authority under the Constitution becomes an abuse of power. When the House Judiciary Committee approved three articles of impeachment against Mr. Nixon for covering up the Watergate burglary and other crimes, it asserted that “he knowingly misused the executive power by interfering with agencies of the executive branch,” including the F.B.I. and the Justice Department.

In effect, the result was to draw a line that subsequent presidents steered clear of, no matter how frustrated they might be at times with investigators pursuing them. To prevent a repeat of Mr. Nixon’s actions, including his order firing the Watergate special prosecutor, Congress enacted a law creating an independent counsel who would not report to the president.

But both parties grew disenchanted with independent counsels after the experience of Presidents Ronald Reagan and George Bush during Iran-contra and Bill Clinton during Whitewater and the Monica S. Lewinsky affair, so they let the law expire. When presidents did use their pardon power in cases close to them — Mr. Bush granting clemency to Caspar W. Weinberger shortly before leaving office and Mr. Clinton doing the same with Susan McDougal in his last hours in the White House — it was considered an egregious violation of the norms.

In the absence of an independent counsel law, Mr. Mueller was appointed as a special prosecutor who still answered to the Justice Department, meaning he had less latitude, could not indict the president even if he thought it merited and could be fired by the president he was investigating, as Mr. Trump threatened to do repeatedly.