(CNN) For months, Democrats probed President Donald Trump and his administration for documents, the President's financial records and interviews with key associates. They ran into one dead end after another .

Now that the investigation has narrowed to Ukraine, that's all changed.

Despite a White House vow that it would not cooperate with the "illegitimate" impeachment inquiry, Democratic investigators have found a way around the stonewalling, obtaining testimony from both current and former Trump administration officials about internal deliberations as well as key text messages between Trump administration officials showing how the administration linked a Ukrainian investigation to a one-on-one meeting between Trump and Ukrainian President Volodymyr Zelensky.

"Whether you want to call it cracks in the armor or a tipping point, when you have an impeachment inquiry in play, people are going to be a little bit more thoughtful as to the implications of their personal character and outcomes that may negatively impact them if they fail to comply with a Constitutionally valid subpoena," said Rep. Harley Rouda, a California Democrat on the House Oversight Committee.

Before the launch of the impeachment inquiry in September, Democrats were operating a multi-pronged investigation strategy with committees each working on their own separate, sometimes duplicate tracks and piling on the administration. The probes included everything from alleged obstruction of justice by the President during special counsel Robert Mueller's investigation to the administration's immigration policies

Now, the three committees leading the impeachment inquiry -- Intelligence, Foreign Affairs and Oversight -- are working in tandem as they've coordinated document and deposition requests spanning nearly every corner of the federal government.

The successful use of subpoenas to compel testimony and handing over of some — though not nearly all — documents to Congress is a welcome change for Democrats who had faced months of White House stonewalling even when they held officials in contempt of Congress for refusing to comply with their subpoenas.

"We have learned much of this thanks to the courageous testimony of State Department officials who have been put in an impossible situation by the administration and that is urged not to comply with the law," House Intelligence Chairman Adam Schiff, a California Democrat, said Tuesday. "They are doing their duty. And people should make no mistake about that. They are doing what they are required to do, and I think showing enormous courage."

Hurdles remain

Democrats still face key roadblocks in their investigation. The Trump administration did not comply with document request deadlines Tuesday for the Office of Management and Budget, the vice president or the Department of Defense. And, Rudy Giuliani, the President's personal lawyer, also announced he wouldn't turn over documents for the impeachment inquiry.

"Were it not for the fact that at least some witnesses have given us documents, we would not know that there is a paper record of efforts to condition this meeting and perhaps condition military support itself on these political investigations," Schiff said. "We know from the additional witnesses who have come forward that there are additional documents that they have provided the State Department but have not been given to Congress."

Yet, Democrats feel they've made huge strides in their investigation in the last two weeks compared to past efforts.

In its impeachment investigation, the House Judiciary Committee struggled to obtain testimony from Trump administration witnesses even after they had left the administration. When they did, the witnesses refused to answer questions about their time at the White House. In one extreme example, former White House Communications Director Hope Hicks refused to even tell investigators where her West Wing office was located because it pertained to her time in the White House.

But following the whistleblower complaint released in last month alleging that Trump had enlisted Ukraine's help to dig up dirt on his potential 2020 opponent, the White House wall blocking congressional investigations has started to crack.

The House Intelligence Committee-led investigation into Ukraine has obtained testimony from, former US Special Envoy to Ukraine Kurt Volker, former top White House Russia adviser Fiona Hill and former US Ambassador to Ukraine Marie Yovanovitch , who remains a State Department employee.

Hill appeared Monday under subpoena before the three committees, where she told lawmakers behind closed doors that former national security adviser John Bolton described the President's personal lawyer Rudy Giuilani as a "hand grenade" and said the characterized an operation carried out by US Ambassador to the European Union Gordon Sondland and White House acting chief of staff Mick Mulvaney as being like a "drug deal."

On Tuesday, State Department Deputy Assistant Secretary of State George Kent appeared behind for closed-door testimony after receiving a subpoena, and three more current and former administration officials from State and the Pentagon are scheduled for the rest of the week — including Sondland, who has said he will testify after being subpoenaed.

Why this time is different

There are several key reasons why the Ukraine investigation is seeing more success compared with previous investigations. In some ways, it's due to the fact that the allegations themselves seem clearer — that the President used his office to leverage election interference from a foreign government. The origin of those allegations is a whistleblower complaint has been corroborated by the transcript of the call between Trump and Ukraine's President, provided by the White House. Lawmakers argue there is also an urgency in that the 2020 election is just around the corner.

"Investigating 2016 in 2019 in some ways is looking back and some people argued well you are trying to re-litigate an election that's already over," said Rep. Eric Swalwell, a Democrat on the Intelligence Committee. "Here, the key is talking about a future election and so it is like you have the opportunity to (be) sure you have a pure election. That weighs on people."

But perhaps most importantly for the Democrats' probe, former officials have not been persuaded by the White House's vow not to cooperate with the investigation, particularly when receiving a subpoena.

That wasn't the case with Don McGahn, a key witness in Mueller's obstruction investigation, who defied a subpoena from the Judiciary Committee and did not testify. The committee is currently suing to force him to appear.

The White House also successfully prevented any witnesses from turning over documents or testify about events by claiming the right to invoke executive privilege, but that hasn't been as successful this time around.

Volker, who resigned a week before he appeared , appeared voluntarily and brought documents showing encrypted messages he exchanged with Sondland and other State Department officials, Ukrainian political aides and Giuliani — texts that are likely to form a key part of the Democrats' case for impeachment.

Hill's lawyer in a letter to the White House ahead of her testimony pushed back on the White House's executive privilege claims, including noting legal rulings that executive privilege over deliberations "disappears altogether when there is any reason to believe government misconduct occurred."

Yovanovitch's interview remained in doubt until the morning of her testimony , when she arrived on Capitol Hill several minutes after the deposition was supposed to begin. But her appearance has now set a precedent for witnesses who are appearing before the committees investigating impeachment. After the White House directed her not to appear, the committees responded with a subpoena to compel her appearance, which she complied with.

Both Hill and Kent have followed the same formula, receiving a subpoena before their testimony.

That doesn't mean Democrats will get every witness they schedule, of course. State Department Counselor Ulrich Brechbuhl's scheduled deposition has disappeared from the committee's calendar this week without explanation. And the associates of Rudy Giuliani have shown no indication they will appear, although one associate is being cooperative with the committee and says he has no information to offer.

But Democrats have also adopted a different strategy to combat the defiance of subpoenas. With their previous investigations, they moved to hold witnesses and Cabinet officials in contempt of Congress, and then took their cases to court, where they're still battling with Trump to obtain Mueller's grand jury information, Trump financial documents, Trump's tax returns and the testimony of McGahn.

It's not clear when those cases will be resolved.

For the Ukraine probe, however, Democrats have echoed a simple refrain: any efforts to stonewall the investigation will become evidence of obstruction of Congress, and are likely to be written into potential articles of impeachment.