“Thank you to patriots like @realDonaldTrump appointee Fiona Hill who chose to ignore the obstruction from Trump and gave testimony to Congress today,” said Rep. Ted Lieu (D-Calif.). “The truth will keep coming out. And Trump cannot stop it.”

In closed-door testimony described by a source in the room, Hill said she raised concerns with White House officials over Giuliani’s campaign to pressure Ukrainian officials to probe Trump’s political rivals.

Hill said she shared her concerns with then-national security adviser John Bolton, who encouraged her to report her concerns about Giuliani's efforts to a National Security Council lawyer. She told House impeachment investigators that she met with the lawyer, John Eisenberg, twice. Hill also connected Giuliani's efforts to Trump's acting chief of staff, Mick Mulvaney, and said Bolton characterized their efforts on Ukraine as a “drug deal.”

According to a source in the room Monday, Hill said Bolton compared Giuliani to “a hand grenade who’s going to blow everybody up.”

And the flood of damaging information isn’t subsiding.

As lawmakers returned to Capitol Hill on Tuesday, a growing number of witnesses are poised this week to describe their own roles in the controversy, even as the White House has vowed not to engage with House Democrats’ “illegitimate” impeachment effort.

On Wednesday, Michael McKinley, who abruptly resigned last week as a top aide to Secretary of State Mike Pompeo, intends to testify before lawmakers.

On Thursday, lawmakers are expected to hear from Gordon Sondland, the U.S. ambassador to the European Union whose text messages revealed by lawmakers indicated he was aware of efforts to pressure Ukrainian officials to investigate Biden. Sondland reportedly is ready to deflect any blame onto Trump about whether there was a quid pro quo involving military aid to Ukraine or a meeting between Trump and his Ukrainian counterpart in Washington.

Congressional investigators on Friday will hear from Deputy Assistant Secretary of Defense Laura Cooper, who oversees Russia- and Ukraine-related matters at the Pentagon.

But despite the series of breakthroughs, Democrats still face resistance from the White House to some of their high-level requests.

When asked whether Trump's budget office had planned to comply with a Tuesday subpoena deadline for documents, a senior administration official would not comment, instead pointing to a White House letter last week that deemed the House impeachment probe “unconstitutional” in part because there has not been a House vote authorizing the inquiry.

Meanwhile, Giuliani, who is facing a mounting set of legal woes, parted ways with his attorney Jon Sale on Tuesday after Sale sent a letter to the three key investigative committees stating that Giuliani would not comply with a congressional subpoena seeking documents. Sale wrote that the subpoena was “overbroad” and “unduly burdensome.”

“Jon has done what I retained him for,” Giuliani told POLITICO.

An official working on the impeachment inquiry slammed the former New York City mayor and said his refusal to comply with the subpoena would be treated as evidence of obstruction and of a cover-up.

“If Rudy Giuliani and the president truly have nothing to hide about their actions, Giuliani will comply — otherwise, we will be forced to consider this as additional evidence of obstruction, and may infer that the evidence withheld would substantiate the accusations of President Trump’s misconduct and efforts to cover it up,” the official said.

Similarly, Vice President Mike Pence rejected House Democrats’ request for Ukraine-related documents, a demand which also had a deadline of Tuesday. Pence’s counsel, Matthew E. Morgan, echoed the White House’s position that the impeachment inquiry is illegitimate. A subpoena to Pence is likely to follow.

The Pentagon, too, rejected a subpoena seeking documents. In a letter to Democratic committee chairs, the Defense Department’s legislative affairs chief also cited the White House’s view on the impeachment inquiry.

But the recent spate of witness interviews underscores how the president’s once-impenetrable barrier to meaningful testimony in Democrats’ impeachment inquiry has been blown apart.

“The walls are closing in. The details we are learning about the shadow foreign policy operation Trump has been running to benefit himself personally are stunning,” said Sen. Chris Murphy (D-Conn.). “Why have a democracy, if we allow this to happen without consequence?”

Though Hill's testimony was the most damning to date, she wasn't the first to put a crack in Trump’s wall.

Earlier this month, former NATO Ambassador Kurt Volker provided text messages between himself and other diplomats in which they described concerns that Trump was using a potential White House visit for Ukrainian President Volodymyr Zelensky, and possibly even military aid, as a cudgel to force the besieged country to probe Biden. Volker testified for nine hours to lawmakers and aides behind closed doors. Trump has forcefully denied any “quid pro quo” occurred.

Last Friday, Marie Yovanovitch, the former U.S. ambassador to Ukraine, testified about her abrupt removal by Trump, which came amid a smear campaign by Trump's allies that accused her of disloyalty. Yovanovitch's ouster in May infuriated senior State Department officials, and she testified that the ability of bad actors to engineer her removal could be exploited by foreign adversaries.

Kent served under Yovanovitch in Ukraine for three years. A former State Department official said Kent is “able to peel back layers of the onion that many people can’t,” and he is likely to speak out against Yovanovitch’s ouster. Connolly said Kent “implicitly” defended Yovanovitch during his testimony.

House Republicans have said little about the substance of Hill's testimony but have complained vehemently about Democrats’ decision to hold witness interviews behind closed doors. They contend a matter as weighty as the potential impeachment of a president should be conducted publicly.

Intelligence Chairman Adam Schiff has countered, arguing that the secrecy surrounding the initial interviews is meant to prevent witnesses from aligning their statements. Speaking at a press conference Tuesday, Schiff said public hearings would soon follow the evidence-gathering phase of the impeachment inquiry.

Caitlin Emma, Darren Samuelsohn and Nahal Toosi contributed to this report.

