But Mr. Turley made only a passing reference in his written statement to the problem that has bedeviled impeachment investigators: The White House has directed top aides to Mr. Trump not to cooperate with the House, while asserting that they are immune from being subpoenaed to testify about their discussions with the president.

Mr. Bolton, for example, has refused to testify unless and until a judge orders him to, and has made clear he will file a lawsuit to put the matter into court if he receives a subpoena rather than deciding for himself whether Congress’s claimed power to compel his testimony, or the president’s claimed power to block lawmakers from receiving it, should prevail.

The Trump legal team’s claim of absolute immunity for top presidential aides has been a losing one in court. A Federal District Court judge already rejected it in a 2008 case involving a congressional subpoena to Harriet Miers, President George W. Bush’s former White House counsel. Another trial judge rejected it again late last month, in a case centering on a subpoena to Donald F. McGahn II, Mr. Trump’s former White House counsel.

But the legal unraveling of Mr. Trump’s argument has been a slow process. The opening stage of the fight over the subpoena to Mr. McGahn consumed nearly a third of the year before the judge completed a 120-page ruling rejecting it. The Justice Department then immediately appealed. It can repeat that process before a three-judge panel, and then again before the full appeals court and then before the Supreme Court.

And even if the Supreme Court ultimately orders such an official to show up for testimony, he could then refuse to discuss conversations with Mr. Trump on the ground that their contents are privileged. That would start a new cycle of litigation.

That means that for the witnesses Mr. Turley identified as having potentially material additional information, the Justice Department would very likely be able to keep the subpoena tied up in court until long after the 2020 election.