While critics argue she undercut the House case with that delay, both Pelosi and House impeachment managers point to the significance of the evidence that's surfaced since the House passed the articles of impeachment Dec. 18.
"Time has been our friend in all this," Pelosi has said, calling the new evidence "incriminating."
Democrats have talked extensively about some of the newly emerged evidence in making their arguments at the Senate trial, but it hasn't yet been formally admitted as they demanded. On a party-line vote, the GOP-led Senate voted to consider whether it should be allowed only later in the trial, after all arguments are made and senators ask written questions. Democrats called that exactly backwards.
Bolton bombshell: Manuscript reportedly says Trump personally tied Ukraine aid to Biden investigation
The New York Times reported Jan. 27 that former national security adviser John Bolton claims Trump told him he was withholding the Ukrainian aid until Ukraine agreed to investigations into former Vice President Joe Biden and his son Hunter.
Bolton reportedly detailed the August 2019 conversation in an unpublished manuscript for an upcoming book. The manuscript was given to the White House for a standard pre-publication review.
Trump denied the report saying, “if John Bolton said this, it was only to sell a book.”
The revelation appeared to undercut the Trump team's legal argument that there is “simply no evidence anywhere that President Trump ever linked security assistance to investigations.”
The report added fuel to the Democrats’ call for witnesses. Four Republicans would need to break ranks and vote with Democrats in order for witnesses to be heard in the Senate trial.
“John Bolton has the evidence,” Schumer tweeted Sunday night. “It’s up to four Senate Republicans to ensure that John Bolton, [acting chief of staff] Mick Mulvaney, and the others with direct knowledge of President Trump’s actions testify in the Senate trial.”
Republican Sen. Mitt Romney is one of four GOP moderates Democrats have targeted in hopes of getting their support for witnesses. He said Monday it’s “important” senators hear Bolton’s account.
“It’s pretty fair to say that John Bolton has relevant testimony to provide to those of us who are sitting in impartial justice,” he said.
Shortly after Romney spoke to reporters, GOP Sen. Susan Collins, issued a statement on Twitter, saying the reports on Bolton’s book “strengthen the case for witnesses and have prompted a number of conversations among my colleagues.”
“I’ve always said that I was likely to vote to call witnesses, just as I did in the 1999 Clinton trial,” she said in the tweet.
Bolton says he's willing to testify if subpoenaed
Bolton was one of the four individuals Schumer demanded the Senate call as witnesses, noting his attorney previously told House lawyers Bolton was “personally involved in many of the events, meetings, and conversations about which you have already received testimony, as well as many relevant meetings and conversations that have not yet been discussed in the testimonies thus far.”
Schumer called Bolton's statement "momentum for uncovering the truth."
"Given that Mr. Bolton’s lawyers have stated he has new relevant information to share, if any Senate Republican opposes issuing subpoenas to the four witnesses and documents we have requested they would make absolutely clear they are participating in a cover up," he said.
Pelosi echoed Schumer on Twitter.
"The President & Sen. McConnell have run out of excuses. They must allow key witnesses to testify, and produce the documents Trump has blocked, so Americans can see the facts for themselves," she said.
91 minutes between Ukraine phone call and withholding of aid
On Dec. 22, documents obtained by the Center for Public Integrity revealed White House officials requested almost $400 million in aid to Ukraine be held 91 minutes after Trump’s call with Ukrainian President Volodymyr Zelenskiy.
According to a record of the call released by the White House, the July 25 phone call between Trump and Zelenskiy took place between 9:03 a.m. and 9:33 a.m.
At 11:04 a.m., Mike Duffey, an official with the White House’s budget office, sent an email to Deputy Secretary of Defense David Norquist, the chief of staff to Defense Secretary Mark Esper and the Pentagon's chief financial officer. In that email, he instructed them to put a hold on the aid.
A spokesperson for the Office of Management and Budget responded to the revelation by saying, “to pull a line out of one email and fail to address the context is misleading and inaccurate.”
Senate Minority Leader Chuck Schumer called the new evidence "explosive," pushing his case for Duffey and other key players to be called as witnesses at the Senate trial.
"A top administration official, one that we've requested, is saying 'stop the aid' 91 minutes after Trump called Zelenskiy, and said 'keep it hush-hush," Schumer said. "What more do you need to request a witness?"
Unredacted email says aid freeze was at 'clear direction of POTUS'
On Jan. 2, an unredacted version of an email disclosed Duffey ordered the Pentagon to keep the freeze in U.S. assistance to Ukraine at the “clear direction from the POTUS.”
Rep. Adam Schiff, the lead House prosecutor in the Senate impeachment trial, said these emails undermined the White House’s argument that the aid was held for a legitimate reason. U.S. law provides exemptions for the executive branch to withhold spending if the administration informs Congress of such decisions on a timely basis, which was not the case with the Ukranian aid.
"The documents show a compelling desire to prevent Congress from finding out,” Schiff said in a statement. “If there was a legitimate reason to place the hold and there was no concern about violating the law, they would have told Congress. But of course they did not, since the whole point of the aide freeze was to coerce Ukraine into interfering in our election to help the president.”
Despite Democrats' call for witnesses, Senate Majority Leader Mitch McConnell pressed for a swift trial without them. The Senate ultimately rejected Schumer's proposed amendment to the Senate trial rules, which would have subpoenaed testimony from Duffey. The amendment failed along party lines.
In a tweet, Pelosi said the emails showed an “unprecedented, total obstruction of Congress” by the president.
Parnas says ‘Trump knew exactly what was going on’ with Ukraine
In an interview on MSNBC’s “"The Rachel Maddow Show",” Lev Parnas, an indicted associate of the president’s personal attorney, Rudy Giuliani, claimed President Trump was fully aware of Giuliani's efforts in Ukraine.
In an interview on CNN the following evening, Parnas said both his and Giuliani’s efforts in Ukraine were “all about 2020, to make sure [Trump] had another four years.”
Parnas participated in the first of these interviews on Jan. 15, the same day Pelosi officially began the process to send the articles of impeachment to the Senate.
Shortly after delivering the articles, the House Intelligence Committee published new records gathered from Parnas as part of a subpoena request from September. The records included voicemails, photographs, videos and messages further linking Giuliani to his involvement in Ukraine -- a key part being the efforts to dismiss former U.S. Ambassador to Ukraine Marie Yovanovitch.
In one text exchange from March 2019, a Connecticut man and a Trump donor named Robert F. Hyde texted Parnas: "Wow. Can't believe Trump hasn't fired that bitch. I'll get right in that."
The texts came one month before Yovanovitch was fired from her post, but Parnas and Hyde dismissed the exchange as not serious.
The president adamantly denied ever knowing much about Parnas and what he was up to with Giuliani, to which Parnas responded he was “lying.”
'Take her out': Recording appears to capture Trump telling associates to fire Yovanovitch
ABC News obtained a recording that appeared to capture Trump telling associates at an intimate dinner he wanted the then-U.S. ambassador to Ukraine Marie Yovanovitch fired.
Sources familiar with the recording said it was made during a dinner at the Trump International Hotel in Washington, D.C. on April 30, 2018. Attendees at the dinner included Parnas and Igor Fruman, e both former business associates of Rudy Giuliani.
“Get rid of her!” the voice that appears to be Trump is heard saying. “Get her out tomorrow. I don’t care. Get her out tomorrow. Take her out. OK? Do it.”
Before the comment is made, Parnas appears to say: “The biggest problem there, I think where we need to start is we gotta get rid of the ambassador. She’s still left over from the Clinton administration. She’s basically walking around telling everybody, ‘Wait he’s gonna get impeached, just wait.”
A year later, in April 2019, Yovanovitch was recalled from her position. She said the decision was based on “unfounded and false claims by people with clearly questionable motives” that she was disloyal to Trump.
White House press secretary Stephanie Grisham denied any wrongdoing by the president.
“Every president in our history has had the right to place people who support his agenda and his policies within the Administration,” she said.
A copy of the recording is now in custody of the federal prosecutors in New York’s Southern District. They declined to comment to ABC News.