President Trump said in a tweet that he will release on Wednesday a transcript of his July phone call to Ukrainian President Volodymyr Zelensky, in which they reportedly discussed an investigation involving Joe Biden.

"I am currently at the United Nations representing our Country, but have authorized the release tomorrow of the complete, fully declassified and unredacted transcript of my phone conversation with President Zelensky of Ukraine," Trump wrote. "You will see it was a very friendly and totally appropriate call. No pressure and, unlike Joe Biden and his son, NO quid pro quo! This is nothing more than a continuation of the Greatest and most Destructive Witch Hunt of all time!"

Trump added: "The Democrats are so focused on hurting the Republican Party and the President that they are unable to get anything done because of it, including legislation on gun safety, lowering of prescription drug prices, infrastructure, etc. So bad for our Country!"

Department of Justice lawyers, as well as lawyers at the White House, have been advising White House officials to release the transcript since last week, a source familiar with the conversations told Fox News.

The dramatic "flip-the-script" moment could blunt Democrats' newly energized push to begin a formal impeachment inquiry, based on a whistleblower's allegation that the president pressed the new Ukrainian leader during the phone call to investigate the former vice president and his son Hunter.

But Democrats were sure to continue their push for the whistleblower's full complaint and testimony, which could provide additional details. House Intelligence Committee Chairman Adam Schiff, D-Calif., said Tuesday afternoon that testimony from the whistleblower might be imminent.

"We have been informed by the whistleblower’s counsel that their client would like to speak to our committee and has requested guidance from the Acting DNI [Director of National Intelligence] as to how to do so," Schiff said in a tweet. "We‘re in touch with counsel and look forward to the whistleblower’s testimony as soon as this week."

It was unclear if Acting DNI Joseph Maguire would provide that guidance. Maguire is expected to testify himself before the House Intelligence Committee at an open hearing on Thursday -- but Maguire has refused to share some information, including the complaint.

A source familiar with the matter told Fox News this week that the whistleblower did not have “firsthand knowledge” of the conversation between Trump and the Ukrainian president.

And Yahoo News reported that the GOP-controlled Senate Intelligence Committee had written to the whistleblower's attorney, requesting a bipartisan closed-door interview.

At the same time, the matter threatened to shine a harsh spotlight on the Bidens' activities in Ukraine, and top Republican Rep. Devin Nunes, R-Calif., warned the situation could "backfire" on Democrats.

"The longer we talk about what the Bidens did in Ukraine, the better," said Barry Bennett, a former Trump campaign adviser, who dismissed those who believe Trump will pay a political price for the latest controversy.

Joe Biden has acknowledged that, when he was vice president, he successfully pressured Ukraine to fire its top prosecutor, Viktor Shokin, while Shokin was investigating the natural gas firm Burisma Holdings — where Hunter Biden was on the board.

Top Trump surrogates, including his personal attorney Rudy Giuliani, have long said that Ukraine should investigate that matter fully.

And Trump has long said he wants European countries to pay more for their own defense. An administration official told the Wall Street Journal that Trump's actions on the call with Zelensky in July "reflected the president’s concerns about how the U.S. is spending aid money and whether U.S. allies are adequately contributing."

In remarks to reporters at the United Nations on Monday, Trump denied linking the aid money to Ukraine's investigative actions.

“No, I didn’t — I didn’t do it,” Trump said. But he also repeatedly called the Bidens' actions in Ukraine a "disgrace," and added: “It’s very important to talk about corruption. ... Why would you give money to a country that you think is corrupt?”

Still, there were other indications that releasing the transcript would not fully satisfy the growing chorus of Democrats calling for an impeachment inquiry. In a statement on Sunday, House Speaker Nancy Pelosi, D-Calif., strongly suggested that impeachment might be on the table if the whistleblower's full complaint was not provided to Congress.

"If the Administration persists in blocking this whistleblower from disclosing to Congress a serious possible breach of constitutional duties by the President, they will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation," Pelosi said.

She was expected to get behind an impeachment inquiry on Tuesday.

Several commentators also cast skepticism on Trump's announcement within minutes.

"Nixon released his own transcripts of the Oval Office tapes — which turned out to be wildly misleading," said historian Kevin Cruse.

Added Politico correspondent Natasha Bertrand: "Full transcript ≠ full whistleblower complaint."

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Meanwhile, a slew of key swing-district Democrats late Monday threw their support en masse behind opening a formal impeachment inquiry, as The New York Times, The Washington Post and The Wall Street Journal reported in the evening that the president personally ordered acting White House chief of staff Mick Mulvaney to freeze nearly $400 million in military aid to Ukraine just days before his phone call with the Ukrainian president.

Fox News' Jake Gibson and The Associated Press contributed to this report.