[15][16] Investigations into various scandals in the Trump administration, which could lead to articles of impeachment, were initiated by various House congressional committees led by Pelosi in February 2019.

[18] The Trump–Ukraine scandal revolved around efforts by former U.S. president Donald Trump to coerce Ukraine and other foreign countries into providing damaging narratives about 2020 Democratic Party presidential primary candidate Joe Biden, as well as information relating to the origins of previous political attacks against him, such as the claims investigated by Robert Mueller.

Trump enlisted surrogates within and outside his official administration, including his lawyer Rudy Giuliani and Attorney General William Barr, to pressure Ukraine and other foreign governments to cooperate in investigating conspiracy theories concerning American politics.

[19][20][21][22][23] Trump blocked but later released payment of a congressionally mandated $400 million military aid package to allegedly obtain quid pro quo cooperation from Volodymyr Zelenskyy, the president of Ukraine.

[19][20][21][24] Less than two hours later, on behalf of the President, senior executive budget official Michael Duffey discreetly instructed the Pentagon to continue withholding military aid to Ukraine.

A non-verbatim transcript of the Trump–Zelenskyy call confirmed that Trump requested investigations into Joe Biden and his son Hunter, as well as a discredited conspiracy theory involving a Democratic National Committee server,[30][31] while repeatedly urging Zelenskyy to work with Giuliani and Barr on these matters.

[38] United States ambassador to the European Union Gordon Sondland testified that he worked with Giuliani at Trump's "express direction" to arrange a quid pro quo with the Ukraine government.

[39] On the evening of September 24, 2019, Pelosi announced that six committees of the House of Representatives would begin a formal impeachment inquiry into President Trump.

He walked back his comments later that day, asserting there had been "absolutely no quid pro quo" and that Trump had withheld military aid to Ukraine over concerns of the country's corruption.

[56] Later that day, Kurt Volker, the former U.S. special representative for Ukraine, and Tim Morrison, the former national security presidential adviser on Europe and Russia, gave public testimony before the House Intelligence Committee.

[58] On November 20, 2019, Ambassador Sondland testified that he conducted his work with Giuliani at the "express direction of the president",[59] and that he understood a potential White House invitation for Zelenskyy to be contingent on Ukraine announcing investigations into the 2016 elections and Burisma.

"[79] Nadler responded in a statement, "We gave President Trump a fair opportunity to question witnesses and present his own to address the overwhelming evidence before us.

The witnesses invited by Democrats were law professors Noah Feldman from Harvard, Pamela S. Karlan from Stanford, and Michael Gerhardt from the University of North Carolina.

One of the highlights of this contentious event was Georgia representative Barry Loudermilk comparing the impeachment inquiry of President Trump to the trial of Jesus Christ, saying that the Christian savior was treated far better by the authorities.

[124] On Christmas Day, he tweeted: Why should Crazy Nancy Pelosi, just because she has a slight majority in the House, be allowed to Impeach the President of the United States?

[125]Attorney George T. Conway III and others noted that if the relevant witnesses were not allowed to testify, Trump's defenders would be negatively affected by "the very evidence they sought to suppress".

[135][136] Two days later, McConnell rejected the call for witnesses to testify, saying that the Senate's role is simply to act as "judge and jury" and not to aid the impeachment process.

[139][142] On January 2, 2020, Schumer called newly unredacted emails from Trump administration officials "a devastating blow to Senator McConnell's push to have a trial without the documents and witnesses we've requested".

[146] Rudy Giuliani stated his willingness to testify or even try the impeachment "as a racketeering case", despite being Trump's personal attorney and allegedly attempting to help him politically while searching for evidence against the Bidens in Ukraine.

[147] On January 10, 2020, Trump told Laura Ingraham of Fox News that he would likely invoke executive privilege to keep Bolton from testifying "for the sake of the office".

[158] On January 10, Pelosi announced she had "asked Judiciary Committee Chairman Jerry Nadler to be prepared to bring to the Floor next week a resolution to appoint managers and transmit articles of impeachment to the Senate".

Later that afternoon, Pelosi held a rare public engrossment ceremony, followed by a stately procession of the managers and other House officers across the Capitol building, where the third impeachment of a U.S. president was announced to the Senate.

McConnell added that the coordination with the White House would also pertain to whether witnesses would be allowed to testify,[166][167] and told Sean Hannity of Fox News that there was no chance Trump would be convicted, expressing his hope that all Senate Republicans would acquit the president of both charges.

[182][183][184] On the first day, Schumer called the previous evening "a dark night for the Senate", when the White House, in response to a Freedom of Information Act lawsuit, released new evidence including a string of heavily redacted emails revealing details about how the Office of Management and Budget froze aid to Ukraine.

[186][q] Professor Alan Dershowitz argued that while a president can be impeached for committing a criminal act, irrespective of motive, the idea of a quid pro quo being a basis for removal from office requires that the 'quo' be something illegal, and that simply having mixed motives for requesting a legal act (an investigation into alleged corruption) would not be sufficient grounds for impeachment.

[189] On January 31, after a planned debate session, the Senate voted 51–49 against allowing subpoenas to call witnesses, including former national security advisor John Bolton,[r] or documents.

[198] A USA Today / Suffolk University poll conducted between December 10 and 14, 2019, found that 45% of respondents supported the impeachment and removal of Trump from office, while 51% opposed it.

[202] An NBC / Wall Street Journal poll released on January 2, 2020, showed 46% favored removal from office and 49% opposed, with the in favor/opposed being almost exclusively along party lines.

[216][217] On February 7, Gordon Sondland's ambassadorship was terminated, and Lieutenant Colonel Alexander Vindman was escorted from the White House after a dismissal from his job on the National Security Council.

[220] Atkinson responded that he believed Trump had fired him for "having faithfully discharged my legal obligations as an independent and impartial inspector general, and from my commitment to continue to do so".