Pointed inquiries are expected from Republicans of the lead impeachment manager, Rep. Adam B. Schiff (D-Burbank), including whether he should have pushed harder to get Bolton's testimony during the House investigation.
Both sides had 24 uninterrupted hours to make their case to senators.
Democrats accuse the president of pressuring Ukraine to investigate political rivals, using a White House meeting and nearly $391 million in military aid as leverage, to boost his 2020 re-election chances. In a Senate split 53-47 in favour of Republicans, at least four Republican senators must join all Democrats to reach the 51 votes required to call witnesses, decide whom to call or do almost anything else in the trial. They also argued that Trump has not committed any crime and therefore can not be removed from office, even though many legal scholars say breaking a law is not required in order to secure an impeachment conviction that would remove a president from office.
But Republicans threatened to demand that Democratic presidential hopeful Joe Biden and his son Hunter also testify, in a move aimed at damaging the party's possibly best chance of defeating Trump in the November election.
If all Democrats vote in favour of that resolution, as is expected, they'd still need four Republicans to join them to reach the simple majority (51 votes) needed to pass it.
Engel, a New York Democrat, said that his call with Bolton was 'one of the reasons we wished to hear from Ambassador Bolton, under oath, in a formal setting'. Trump has said he fired Bolton.
After this week's early leaks about the book, White House aides and allies are privately expressing concern about what more Bolton might reveal that the president and others in his orbit would find embarrassing.
While Engel said in his statement that he informed investigative colleagues, House Speaker Nancy Pelosi's office told CNN she was not aware of the conversation before a formal impeachment inquiry was announced on September 24, the day after the Engel-Bolton call occurred.
Republicans lack the votes to block witnesses at President Donald Trump's impeachment trial, Senate Majority Leader Mitch McConnell conceded late Tuesday, a potentially major hurdle for Trump's hopes to end the trial with a quick acquittal.
Afterward, Republican senators met behind closed doors to discuss calling witnesses including Bolton, but said as they emerged that there was no resolution on the matter. "You know what the right answer is in your heart", White House Counsel Pat Cipollone stated. The sessions come after six days of oral arguments over Trump's impeachment by the House. Sekulow also sought to undermine the credibility of Bolton's book by noting that Attorney General William Barr has disputed comments attributed to him by Bolton.
They tackled the allegations on Wednesday - that Mr Trump told Mr Bolton he wanted to keep freezing military aid for Ukraine until he got help with investigations of his political rivals - head on. The news about the manuscript, first described by The New York Times over the weekend, caught Senate Republicans by surprise, upping the pressure on them to allow witnesses such as Bolton and other documents to be presented during the trial.
The argument built on a separate one Monday night from Trump attorney Alan Dershowitz, who said that nothing in the manuscript - even if true - rises to the level of an impeachable offense.
"I'd like to hear from John Bolton". And now that the President has called his credibility into question, it's important to set the record straight, Engel said. They apparently fear that the established process will not give them what they want, so they attempt to create a different process.