Cracks are starting to emerge in the White House's overarching strategy not to cooperate with any aspect of House Democrats' impeachment investigation and other probes.

In the course of a few hours Friday, former U.S. Ambassador to Ukraine Marie Yovanovitch appeared on Capitol Hill to testify, defying the White House's order that she skip the closed-door deposition. Gordon Sondland, U.S. ambassador to the European Union, indicated he will testify on Thursday, after the State Department blocked his appearance this week.

And a federal appeals court ruled Friday in favor of Democrats seeking Trump's tax returns and other financial records - documents the president has refused to turn over.

After watching their investigations get bogged down for nearly a year by Trump's stonewalling tactics, Democrats' aggressive oversight efforts are finally beginning to bear fruit.

In a letter to Democratic colleagues Friday, Speaker Nancy Pelosi (D-Calif.) heralded the "major victory" in the Mazars case, where the D.C. Circuit Court ruled that Trump's accounting firm would need to comply with a House Oversight and Reform Committee subpoena demanding financial records from Trump and his business entities.

The Speaker also called attention to a joint statement from 17 former Watergate prosecutors who called on the House to continue carrying out the Trump impeachment investigation "fairly, openly and promptly."

"This week, we have seen increased outside validation of our efforts to hold the President accountable and these statements speak to the heart of the Constitutional challenge that we face," Pelosi wrote to her colleagues.

The court ruling and testimony from Trump administration officials have given Democrats investigating Trump a boost of momentum, just as lawmakers prepare to return to Washington after a 17-day recess.

These developments also have poked holes in the executive branch's broad strategy to simply stonewall congressional investigators. The White House counsel Pat Cipollone fired off a letter to Democrats this week stating that no one from the Trump administration would testify or participate in what he called Democrats' "partisan and unconstitutional" impeachment probe.

Kurt Volker, Trump's former special envoy to Ukraine, last week became the first witness to testify in the investigation, which is centered on a July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky. A government whistleblower has alleged that Trump pressured Zelensky to investigate corruption allegations against former Vice President Joe Biden, a leading 2020 presidential contender, and appeared to threaten to withhold almost $400 million in U.S. military aid if Zelensky didn't comply.

Trump has forcefully denied any quid pro quo. But Volker handed over text messages that showed one top U.S. diplomat, William Taylor, raising serious concerns to Volker and Sondland that Trump was soliciting help from a foreign leader to boost his reelection chances.

Taylor called that arrangement "crazy," though Sondland replied that Taylor was "incorrect" there was a quid pro quo. On Friday, Sondland's attorneys said despite the State Department's order not to testify, their client will abide by the congressional subpoena and appear before the Intelligence Committee. He will not, however, turn over any documents, which his attorneys said belong to State.

Separately, Democrats are also now interested in deposing Taylor, the U.S. chargé d'affaires of Ukraine, who had threatened last month to quit over the Ukraine affair. They've also issued a subpoena to Energy Secretary Rick Perry. And they've scheduled depositions with other past and present Trump administration officials as well, including Fiona Hill, a former staffer on the National Security Council who left the White House in August.

"We expect to announce additional testimony from relevant witnesses in the coming days and remain prepared to compel testimony through duly authorized subpoenas as appropriate," Intelligence Committee Chairman Adam Schiff (D-Calif.) wrote in a letter to colleagues Friday.

By Friday morning, it was still unclear if Yovanovitch would show at the Capitol given the blanket order by the White House. After she did appear, the Democratic chairmen of three House investigating committees - Schiff, Elijah Cummings (Md.) and Eliot Engel (N.Y.) - issued a statement revealing that the Trump administration had tried to block her deposition. They said she appeared only after Democrats sent her a subpoena - just hours before the scheduled hearing - compelling her to testify.

"This duly authorized subpoena is mandatory, and the illegitimate order from the Trump Administration not to cooperate has no force," the trio of chairmen wrote.

"Any efforts by Trump Administration officials to prevent witness cooperation with the Committees will be deemed obstruction of a co-equal branch of government and an adverse inference may be drawn against the President on the underlying allegations of corruption and coverup," they wrote.

Yovanovitch, a career diplomat, made clear that she felt she'd been wronged by the administration in being recalled to Washington. The administration, she testified in her opening statement, had conducted "a concerted campaign" against her.

Anticipating a critical witness, the White House had sent a set of talking points to Capitol Hill as a line of defense. The talking points asserted that administration officials were "not concerned" with anything Yovanovitch might say, "because the President did nothing wrong."

The talking points were intended for Trump's Republican allies, but were mistakenly also sent to Democrats on the House Foreign Affairs Committee.

White House adviser Stephen Miller said there are not concerns at the White House that their strategy to refuse to cooperate with the impeachment inquiry isn't working.

"I don't think at all," Miller told reporters at the White House Friday afternoon.

"I think Adam Schiff and I think the entire House Democratic caucus is humiliating and embarrassing themselves. They're running these secret sham proceedings, a violation of every principle of justice, due process and fairness that is embedded in our constitution and our cultural history and centuries of legal precedent," he said. "They are hopelessly embarrassing themselves in witch hunt part two."