WATCH: Acting DNI Director Testifies About Whistleblower Complaint

The nation's top spy is set to face Congress on Thursday as Washington hurtles toward another milestone in the fast-deepening Ukraine affair.

Acting Director of National Intelligence Joseph Maguire is scheduled to testify about the origins of the flap at 9 a.m. ET before the House Permanent Select Committee on Intelligence.

He then has a closed session in the afternoon before the Senate Select Committee on Intelligence.

Watch the House hearing here as it begins.

Listen to NPR Special Coverage below:

The House hearing will be watched closely in a capital roiling after House Democrats reached a new consensus this week about moving ahead with an impeachment inquiry — although no one knows what that may mean in practical terms or whether it will bring real consequences for Trump.

The saga began with a complaint to the intelligence community's internal watchdog about allegedly improper commitments made by President Trump to a foreign leader.

That led, on Wednesday, to the extraordinary release by the White House of the account of a call in which Trump asked Ukraine's president to investigate the family of former Vice President Joe Biden.

What still isn't clear is what the whistleblower complaint — if it became public — might add to the public understanding about the Ukraine case. The intelligence committees received partial, still-classified copies of the document on Wednesday.

Rep. Eric Swalwell, D-Calif., who sits on the House intelligence panel, told NPR on Wednesday that the materials he reviewed were "alarming" — but incomplete.

Swalwell said the committee still must hear from the whistleblower in person and see other documents that so far have not reached Congress, including those related to the inspector general's investigation.

"We will ask for more," he said. "We're entitled to more."

How much can DNI discuss?

Maguire has vowed, in general terms, that he'll do his duty in handling the Ukraine affair — but he also denied a press report on Wednesday that he had threatened to resign if he was constrained from talking with lawmakers.

"At no time have I considered resigning my position since assuming this role on Aug. 16, 2019," he said. "I have never quit anything in my life, and I am not going to start now."

Maguire found himself in the job following Trump's removal of the Senate-confirmed former DNI, Dan Coats, and the resignation of his deputy, Sue Gordon. There is no word as to when the White House might nominate their full-time replacements.

So in the meantime, Maguire is caught in the midst of the latest contretemps among Trump, the intelligence community and a foreign government. And Maguire may face questions on Thursday that go beyond the nature of the complaint to the top intelligence inspector general.

President sanguine

Trump told reporters on Wednesday that he had at least one other phone call with Ukrainian President Volodymyr Zelenskiy, as did Vice President Pence — calls that Trump said were also innocent.

Trump said he was willing to release a transcript of those earlier calls, but he was dismissive about the reaction by Democrats.

"Impeachment, for that?" he said. "When you have a wonderful meeting — a wonderful phone conversation?"

The president stuck by his characterization of what he called the real Ukraine story — one he said was about "corruption" involving Biden, his family, Ukraine and China.

Trump responded to a question about whether his request to Ukraine's leader was improper by charging that President Obama and conspirators within the U.S. government had persecuted him in the earlier Russia imbroglio.

Biden has denied any wrongdoing and condemned what he called conspiracy theorizing by Trump and the president's supporters.

The former vice president repeated on Wednesday his earlier call that Trump cooperate with Congress' investigations into the Ukraine affair and said that if Trump doesn't, he'll throw his support behind impeachment.

"It is a tragedy for this country that our president put personal politics above his sacred oath," Biden said. "He has put his own political interests over our national security interests, which is bolstering Ukraine against Russian pressure."