NATIONWIDE — The impeachment inquiry into President Donald Trump is quickly gaining momentum and over the next few days, and a lot of pieces to the puzzle may start falling into place.

On Tuesday, Democrats in the U.S. House of Representatives released a resolution authorizing the next phase of the inquiry. The eight-page document calls for public hearings and requires the intelligence committee to submit a report outlining its findings and recommendations to this point in the process.

The resolution provides "a clear path forward" as the inquiry moves into a more public phase, according to Rules Committee Chairman James McGovern of Massachusetts. 

On Thursday, the House will formally vote on the rules for the impeachment inquiry and to affirm if it moves forward.

Republicans have been calling for the vote since the inquiry was announced by House Speaker Nancy Pelosi.

Democrats are adamant that testimony from officials and the whistleblower show clear signs of quid pro quo when President Trump asked the Ukrainian president to investigate the Bidens.

The complaint at least partially stems from questions over whether the president improperly tried to pressure the Ukrainian president to investigate the son of Democratic presidential candidate Joe Biden.

The president himself has admitted bringing up Biden and his son, Hunter, to Ukrainian President Volodymyr Zelenskiy over the summer. The president has tried to tie Hunter Biden, and by extension his father, to political corruption in the country. 

While vice president under President Barack Obama, Biden led diplomatic dealings with the Ukrainian government. At the same time, his son was serving on the board of a Ukrainian gas company.

Meanwhile, the Trump administration continues to defend the nature of the call in question.

"The transcript of the president's call with President Zelenskiy shows there was no quid quo pro. He did nothing wrong," exclaimed Vice President Mike Pence.

Presidential counselor Kellyanne Conway said, "This fake impeachment inquiry has been ill-conceived and illegitimate."

Also on Thursday, a federal judge wants to hear from lawyers from the White House, the House of Representatives, and from impeachment witness Charles Kupperman.

Kupperman filed a lawsuit, asking the federal court to decide whether he has to testify.

Meanwhile, an Army officer at the National Security Council, who twice raised concerns over the Trump administration's push to have Ukraine investigate Democrats and Biden, arrived in military uniform on Capitol Hill to testify Tuesday in the impeachment inquiry.

Alexander Vindman, an lieutenant colonel who served in Iraq and later as a diplomat, is prepared to tell House investigators that he listened to President Trump's July 25 call with new Zelenskiy and reported his concerns to the NSC's lead counsel, according to prepared testimony.

"I was concerned by the call," Vindman will say, according to the testimony obtained Monday night by The Associated Press. "I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government's support of Ukraine."

Vindman is the first official who listened in on that call to testify as the impeachment inquiry reaches deeper into the Trump administration and Democrats prepare for the next, public phase of the probe. He is also the first current White House official to appear before the impeachment panels.

Vindman, a 20-year military officer and decorated veteran, will testify that he first reported his concerns after an earlier meeting July 10, in which U.S. Ambassador to the European Union Gordon Sondland stressed the importance of having Ukraine investigate the 2016 election as well as Burisma, a company linked to the family of Biden, a 2020 Democratic presidential candidate.

Vindman says he told Sondland that "his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security, and that such investigations were not something the NSC was going to get involved in or push."

That account differs from Sondland's, a wealthy businessman who donated $1 million to Trump inauguration and testified before the impeachment investigators that no one from the NSC "ever expressed any concerns." He also testified that he did not realize any connection between Biden and Burisma.

For the call between Trump and Zelenskiy, Vindman said he listened in the Situation Room with colleagues from the NSC and Vice President Mike Pence's office and was concerned. He said he again reported his concerns to the NSC's lead counsel.

He wrote, "I realized that if Ukraine pursued an investigation into the Bidens and Burisma, it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained. This would all undermine U.S. national security."

Vindman, who arrived in the United States as a 3-year-old from the former Soviet Union, served in various military and diplomatic posts before joining the NSC. He was the director for European affairs and a Ukraine expert under Fiona Hill, a former official who testified earlier in the impeachment probe. Hill worked for former national security adviser John Bolton.

The Associated Press contributed to this story.