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 on Tuesday railed against the impeachment inquiry into his alleged abuse of power ahead of key testimony from a White House official that threatens to deepen the president's problems.

Trump tweeted or retweeted dozens of messages denying wrongdoing, chastising Democrats for their handling of the impeachment proceedings thus far and questioning the credibility of 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 National Security Council official who will meet behind closed doors with lawmakers on Tuesday.

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"Supposedly, according to the Corrupt Media, the Ukraine call 'concerned' today’s Never Trumper witness," Trump tweeted. "Was he on the same call that I was? Can’t be possible! Please ask him to read the Transcript of the call. Witch Hunt!"

Supposedly, according to the Corrupt Media, the Ukraine call “concerned” today’s Never Trumper witness. Was he on the same call that I was? Can’t be possible! Please ask him to read the Transcript of the call. Witch Hunt! — Donald J. Trump (@realDonaldTrump) October 29, 2019

In another tweet, Trump questioned "How many more Never Trumpers will be allowed to testify" and asked "why so many" people were listening in on his July 25 call with Ukrainian President Volodymyr Zelensky.

The president repeatedly urged his followers on Tuesday to read a White House rough transcript of the call, which was released in September. The document shows Trump urging Zelensky to look into Democratic presidential candidate 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 and a company with ties to the Russia investigation.

Vindman on Tuesday will become the first official who was on the call to testify. He will tell lawmakers that he was troubled by Trump urging Zelensky to investigate a political rival and reported it to his supervisor, worrying that the president's conduct threatened to undermine U.S. national security, according to a copy of his opening statement obtained by The Hill.

Vindman is a Ukrainian American immigrant and received the Purple Heart for his service in Iraq.

The July 25 call, a whistleblower complaint about the conversation and testimony from several administration officials have formed the basis of the ongoing impeachment inquiry. The House is scheduled to vote this week to formalize the inquiry and lay out rules to govern the process.

Republicans and White House allies have spent recent weeks hammering Democrats over transparency and questioning the legitimacy of the impeachment inquiry without a formal vote. But in light of Democrats agreeing to hold such a vote, the president's backers have shifted their message.

Trump on Tuesday retweeted dozens of messages from Republican lawmakers and conservative voices blasting the process as a "sham" and disputing that holding a formal vote at this point in the process changes that.

"A vote now is a bit like un-Ringing a bell as House Democrats have selectively leaked information in order to damage President @realDonaldTrump for weeks," Sen. Lindsey Graham Lindsey Olin GrahamLincoln Project mocks Lindsey Graham's fundraising lag with Sarah McLachlan-themed video The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE (R-S.C.) tweeted in one message shared by Trump.

"Codifying a sham process halfway through doesn’t make it any less of a sham process," 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) said in another message the president retweeted.

While Republicans have largely focused their complaints on process, Trump has fixated on the substance of the investigation and repeatedly denied wrongdoing.

"I'd rather go into the details of the case rather than process," Trump said Monday. "Process is good. But I think you ought to look at the case. And the case is very simple. It's quick. It's so quick."

The president's insistence that he has done nothing wrong puts Republicans in a difficult spot, particularly in the Senate, where some GOP lawmakers have been hesitant to defend Trump's actions.

Most Republican senators backed a resolution last week condemning the impeachment inquiry against Trump and calling on the House to hold a formal vote on the inquiry. But the document largely focused on process, and a few key senators have yet to sign on to it in support.