[4] After the emergence of Trump's phone call with Ukrainian president Volodymyr Zelenskyy, House leadership came to the conclusion that impeachment might be advisable, and began an inquiry.

Schumer quickly replied, citing bipartisan public support for the testimony of witnesses who could fill in gaps caused by Trump preventing his staff from testifying in the House investigation.

[19][20] On December 17, McConnell opened the Senate session with a half-hour long speech denouncing the impeachment, calling it "the most rushed, least thorough, and most unfair in modern history", and "fundamentally unlike any articles that any prior House of Representatives has ever passed".

[35] On January 10, Pelosi announced that she and Jerry Nadler were prepared to bring a resolution to appoint managers and transmit the articles of impeachment to the House floor in the next week.

[36][better source needed] On January 6, John Bolton, the former national security advisor in the White House, said he was "prepared to testify" if subpoenaed by the Senate for the impeachment trial.

[37][d] On January 23, as Bolton was preparing for possible Senate testimony about the assertions in the manuscript for his yet-to-be-published book, the National Security Council told him it contained classified information which "may not be published or otherwise disclosed".

[82] The White House and its Senate allies were confident they could garner the simple majority needed to prevent calling witnesses, though they worked on a fallback plan if Bolton was compelled to testify by asserting national security concerns to move his testimony to a closed-door session.

[94] On January 22, the first day of opening arguments, Democrats presented evidence from House impeachment inquiry testimony, the Trump–Zelenskyy phone call and Trump's statements.

"[95] 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.

They also played videos of Fiona Hill and FBI director Chris Wray to debunk the notion that Ukraine, rather than Russia, interfered in the 2016 United States presidential election.

They also pointed out that Vice President Biden pushed out Ukrainian prosecutor Viktor Shokin because he was not fighting corruption in Ukraine, and his ouster was backed by international organizations.

At the end of the day, Susan Collins sent a note to Chief Justice Roberts complaining about Nadler's remarks that senators would be complicit in a coverup if they did not allow testimony from additional witness.

"[121] On January 26, The New York Times reported that Bolton had written in a draft of his forthcoming book that the president told him in August 2019 he wanted to continue freezing $391 million in aid to Ukraine until officials there pursued investigations into Democrats, including the Bidens.

[127] The following day, Pam Bondi[l] dedicated most of her time discussing the motive behind Trump's actions, citing the conspiracy theory involving the Bidens and Burisma, saying, "We would prefer not to be talking about this.

[131] Trump attorney Jane Raskin followed Bondi and told senators, "In this trial ... Mr. Giuliani is just a minor player, that shiny object designed to distract you.

[139] On January 28, the final day of the Trump team's opening statements, Pat Philbin argued that no one could get into the president's mind and decide what is or is not an "illicit motive".

[140] Senators James Lankford (Republican, Oklahoma) and Lindsey Graham proposed that Bolton's book should be reviewed in a classified setting; some argue that this would be illegal.

Senators had to write questions on a sheet of paper addressing the defense, the prosecution or both, which Chief Justice Roberts read out loud for the relevant team to answer.

[150] Dershowitz stirred controversy on January 29 by saying, "If a president does something which he believes will help him get elected in the public interest, that cannot be the kind of quid pro quo that results in impeachment."

[154] Schiff pointed out that during a January 30 federal court hearing about the White House's refusal to honor congressional subpoenas, Judge Randolph Moss asked Justice Department attorney James Burnham what remedies Congress might have in such a case.

[162] The night after the vote, the Justice Department and a lawyer for the Office of Management and Budget acknowledged, in a court filing in a FOIA lawsuit brought against the administration by the Center for Public Integrity, the existence of two dozen emails revealing Trump's thinking about the "scope, duration, and purpose" of the freeze on military aid to Ukraine.

[181] A poll of 1,156 respondents conducted by Social Science Research Solutions in January 2020 found that 51% of people (89% of Democrats, 8% of Republicans, and 48% of independents) supported Trump's removal from office, compared to 45% who opposed the idea.

[101] During the trial on January 24, Senator Marsha Blackburn (Republican, Tennessee) tweeted her disdain for the Director for European Affairs for the United States National Security Council, Lieutenant Colonel Alexander Vindman, questioning his patriotism for testifying against the president.

[190] The White House on January 23 issued a formal threat to stop Bolton from publishing his book, The Room Where It Happened, citing national security concerns, according to CNN.

[191] House Speaker Pelosi said on January 30 that Trump's lawyers had trampled on the Constitution with their arguments that the president could not be impeached for using his office for political gain, and she suggested they should be disbarred.

[192] Two days after Trump's acquittal, she wrote a partisan opinion piece criticizing Republican senators as "accomplices to the president's wrongdoing" who were "normalizing lawlessness and rejecting the checks and balances of our Constitution".

[195] Alexander said the House of Representatives proved the case that Trump withheld military aid to Ukraine to encourage its government to investigate the Bidens, but concluded that the consequences of the president's actions should be decided in the next election.

"[202] He railed against perceived enemies including Mitt Romney, James Comey, and Robert Mueller, and said of Nancy Pelosi and Chuck Schumer, "In my opinion, it's almost like they want to destroy our country.

[214] On February 21, director of the Office of Presidential Personnel John McEntee held a meeting to liaise for coordination among agencies which reportedly included him asking Cabinet liaisons to identify political appointees and career officials working against the President's agenda.

Michael Horowitz, the chair of the Council of the Inspectors General on Integrity and Efficiency, also released a statement rebuking Trump and defending Atkinson's professionalism and standards.