The public phase of the Democrats’ impeachment investigation enters its second week on Tuesday, featuring witnesses both parties think will help to boost their clashing judgments about the propriety of President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's handling of foreign policy in Ukraine.

Lt. Col. Alexander Vindman Alexander VindmanImpeachment witness Alexander Vindman calls Trump Putin's 'useful idiot' The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Strzok: Trump behaving like an authoritarian MORE, a Ukraine specialist on the National Security Council (NSC), has previously relayed dire concerns that Trump’s push to have Ukrainian leaders investigate his political rivals posed a threat to national security — the very allegation at the center of the Democrats' impeachment inquiry.

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Yet Kurt Volker Kurt VolkerGOP senators request details on Hunter Biden's travel for probe Yovanovitch retires from State Department: reports Live coverage: Senators enter second day of questions in impeachment trial MORE, former special envoy to Kyiv, told lawmakers privately last month that the White House decision to withhold military aid was “not significant” and Ukraine's leaders “never communicated a belief that there was a quid pro quo” surrounding the investigations Trump sought, lending a boost to the GOP argument that Trump did nothing wrong.

Both witnesses will appear on Capitol Hill on Tuesday before the rolling TV cameras, along with two other senior officials — Jennifer Williams, a national security veteran detailed to Vice President Pence, and outgoing NSC staffer Tim Morrison — who have also testified previously in the swift-moving impeachment investigation.

While the faces won’t be new to the House Intelligence Committee lawmakers leading the investigation, the public forum will allow voters a wide-open window into the deliberations as both parties wage the critical PR battle over the crucial question at the center of the impeachment inquiry: Did Trump abuse his power in ways demanding his removal?

The White House has readily acknowledged it withheld almost $400 million in aid to Kyiv over the summer, just at the height of the administration’s campaign to press Ukrainian President Volodymyr Zelensky to open investigations into the 2016 elections and the son of former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE. Both of those probes might have helped Trump politically, and Democrats launched their impeachment proceedings after a government whistleblower accused the president of undermining foreign policy for personal political gain.

Vindman was on the now-famous July 25 phone call between Trump and Zelensky — in which Trump asked for the investigations as “a favor” — and he previously testified there was “no doubt” Trump was demanding a quid pro quo. Democrats are sure to highlight those details when Vindman appears on Tuesday.

“Let's be real clear: We are going to have direct evidence this week added to what's already been made public that the president of the United States used taxpayer-funded military assistance to pressure a foreign leader to help him in his reelection campaign,” Rep. Sean Patrick Maloney (D-N.Y.) told CNN on Monday. “That is soliciting a bribe, and that is an impeachable offense listed in the Constitution.”

Volker, while not on the call, was on the front lines of the president’s pressure campaign to get Zelensky to open the sought-after investigations — an effort led by Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE, Trump’s personal lawyer. While Volker expressed concerns with Giuliani’s role in Kyiv, he also suggested the former New York City mayor might also have been freelancing for interests outside those of the president.

In Volker, Republicans see what is perhaps their strongest witness to defend the president from allegations of wrongdoing in Kyiv.

“He said everything that was done here, there was no quid pro of any kind, and it was all done in a way that was consistent with the mission of making sure the aid ultimately gets to Ukraine,” Rep. Jim Jordan James (Jim) Daniel JordanHouse panel pulls Powell into partisan battles over pandemic Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election House passes resolution condemning anti-Asian discrimination relating to coronavirus MORE (R-Ohio), who was shifted to the Intelligence Committee for impeachment's public phase, told CBS's “Face the Nation” on Sunday.

“Ambassador Volker's testimony, I think, will be particularly good and particularly powerful when we get to hear from him later this week,” he added.

The hearing arrives as Democrats are increasingly dismissing the idea that Trump’s allies in Ukraine — such as Giuliani and Gordon Sondland Gordon SondlandGOP chairman vows to protect whistleblowers following Vindman retirement over 'bullying' Top Democrat slams Trump's new EU envoy: Not 'a political donor's part-time job' Trump names new EU envoy, filling post left vacant by impeachment witness Sondland MORE, ambassador to the European Union — were acting as rogue agents in conflict with Trump’s wishes. With that in mind, Democrats are highlighting Trump’s direct role in the push for investigations, including his request to Zelensky for “a favor” and testimony from other witnesses that Sondland spoke directly to Trump about securing the politically tinged investigations.

Morrison had testified that Sondland in September told a top Ukrainian official that the release of U.S. military aid to the besieged country hinged on Kyiv opening the investigations Trump sought.

“Ambassador Sondland believed and at least related to me that the president was giving him instruction,” Morrison testified on Oct. 31.

Tuesday’s jampacked hearing — the most cluttered to date, with four witnesses testifying, two at a time — comes on the heels of last week’s launch of impeachment’s public hearing phase, featuring testimony from three other witnesses embroiled in the Ukraine affair. Senior diplomats William Taylor, George Kent and Marie Yovanovitch Marie YovanovitchGrand jury adds additional counts against Giuliani associates Lev Parnas and and Igor Fruman Strzok: Trump behaving like an authoritarian Powell backs Biden at convention as Democrats rip Trump on security MORE all painted a vivid picture of a shadow foreign policy in Kyiv designed to boost Trump’s political interests — and Giuliani’s business pursuits — at the expense of U.S.-Ukraine relations and efforts to contain Russian aggression in the region.

Vindman and Williams are slated to testify in the morning starting at 9 a.m., followed by Volker and Morrison, who are scheduled to deliver opening statements in midafternoon.

Williams, who was also on the July 25 Trump-Zelensky call, told lawmakers earlier in the month that Trump’s remarks were atypically political for such a conversation. She deemed them “unusual and inappropriate.” The president lashed out on Twitter, saying that if Williams had read the call transcript, then she and other “Never Trumpers” might come up with “a better presidential attack.”

Yet Vindman is perhaps the most visible of the four witnesses. A combat veteran and Purple Heart recipient, he also faced a series of attacks from Trump, who claimed — without proof — that the career civil servant was a “Never Trumper.”

Like Volker, Vindman offered warnings about Giuliani’s involvement in Ukraine policy, telling House investigators last month that he warned other officials that the president’s personal lawyer was a liability — and should therefore be avoided.

Vindman recalled that he told Volker on “a couple occasions” that “there was a lot of risk involved with trying to deal with Mr. Giuliani,” according to the transcript of his Oct. 29 deposition.

And the career official also said he confronted Sondland about making requests for investigations during meetings with Zelensky representatives.

“I stated to Amb. 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,” he said.

Sondland is scheduled to testify, by himself, on Wednesday.