You can tell that the current impeachment inquiry is more threatening to the Trump administration than past congressional oversight, because it has been forced for the first time to conduct itself reasonably with respect to certain witnesses.

Before a few weeks ago, the White House’s consistent strategy had been to stonewall Congress on the basis of, at best, tissue-thin constitutional arguments. It’s a cynical and bad-faith tactic: They know they’re likely to lose, but they’re playing for time — forcing Congress to go to the courts to vindicate basic congressional authority.

That has worked fine so long as the witnesses are willing accomplices in the administration’s contempt for Congress and its disregard for constitutional norms. The White House once carried the day with William P. Barr, Hope Hicks, and even Donald McGahn because those witnesses were willing to go along with its far-fetched arguments.

But now, a number of witnesses have emerged who are ready to do their civic duty and show up in response to lawful subpoenas from Congress. And, with respect to them, the White House finds itself having to quietly play ball and conduct itself like a normal executive branch.

A clear example is Fiona Hill, a former senior official for Russia and Europe on the National Security Council. Hill reportedly told Congress earlier this week that Rudolph W. Giuliani shoved U.S. officials aside to implement a shadow foreign policy aimed to personally benefit President Trump.

That testimony was preceded by the sort of delicate negotiation with the White House counsel that in past congressional inquiries was routine.

Hill’s lawyers met with a deputy White House counsel, Michael Purpura, last week. They explained Hill’s intent to comply with the subpoena. The parties engaged in collegial discussion, after which Purpura wrote Hill’s lawyer a respectful and lawyerly letter.

That letter could not be more different from the unprecedented polemical screed that White House Counsel Pat Cipollone sent congressional leaders last week declaring — with no real legal argument and next to no persuasive rhetoric — that the White House would not cooperate at all with the impeachment inquiry.

With no browbeating, Purpura did his professional best to advance and protect presidential prerogatives. He pushed back, thoughtfully, on arguments from Hill’s lawyer suggesting the limited applicability of executive privilege (acknowledging, for example, that Trump’s disclosure of the contents of the July 25 call with Ukrainian President Volodymyr Zelensky operated as a waiver in the case of that call). He cited law from Democratic and Republican administrations. He neither threatened nor postured; instead, he sought to persuade.

Hill is one of a handful of current or former administration officials who have opted to cooperate with the impeachment inquiry, thus blunting the White House’s strategy of bully and bluster.

More will come. The players in the possible conspiracy with the Russian government during the presidential campaign, which special counsel Robert S. Mueller III investigated, were all Trump partisans. Here, the subordination of normal government channels to the president’s interests necessarily left two kinds of potentially cooperative witnesses: first, officials who were personally involved in the campaign to pressure Ukraine, such as Kurt Volker or Gordon Sondland, and who now hope to get out in front of the scandal and minimize their liability and professional damage; and second, career officials such as Hill or former ambassador Marie Yovanovitch who don’t flinch at their duty to tell the truth when they receive a lawful order.

Better, some of them seem likely to point the finger at one another; that sort of dynamic is fertile ground for skilled investigators.

In the coming weeks, look for a half dozen or more witnesses who can help establish or support the essential allegations from the original whistleblower complaint, and whose testimony the White House will be essentially powerless to prevent. Some, such as William B. Taylor, the acting ambassador to Ukraine, and Michael McKinley, who resigned last week as a senior adviser to Secretary of State Mike Pompeo, are already known.

They are the analogues to witnesses such as Alexander Butterfield, James McCord and John Dean whose congressional testimony created the first large cracks in the Watergate scandal.

House investigators are, wisely, taking what the situation has given them, and the White House is yielding in small degrees, as it must. And while individual witnesses may not be earthshaking, each advances the ball. The upshot is that the House Democrats finally have a bona fide ground game, and are making strides toward the Oval Office every week.

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