“So, I think if there are people that could contribute to this, either innocence or guilt … I think they should be heard.”
He added: “I think some of the conversations seem to me to be very inappropriate, but I wasn’t there.
“But there are people that were there that ought to be heard from.”
Democrats need to convince four Republican senators to vote with them to force witnesses to appear, a move that would potentially extend the impeachment trial for weeks.
Trump’s legal team wrapped up their oral arguments on Tuesday (Wednesday Australian time), arguing senators should not overturn the results of the last election with just months remaining until Americans go to the polls again.
“End the era of impeachment for good,” Pat Cipollone, Trump’s White House counsel, said. “You know it should end.”
Trump’s lawyer Jay Sekulow said: “The bar for impeachment cannot be set this low. Danger, danger, danger. These articles must be rejected. The constitution requires it. Justice demands it.”
He continued: “The President has constitutional authority to engage, conduct foreign policy and foreign affairs.
“It is our position, legally, the President at all times acted with perfect legal authority, inquired of matters in our national interest.”
Democratic impeachment manager Adam Schiff said: “Trump’s lawyers cannot, and did not, defend him on the facts. Their defence has evolved to this: he did it, so what?
“His lawyers strengthened our case that the Senate must hear from Bolton. A fair trial requires key witnesses. Will America have one?”
The trial will now move to two days’ worth of questions and answers, in which senators will be able to submit queries to the Democratic impeachment managers and Trump’s legal team for a response.
Senate Majority Leader Mitch McConnell told senators privately on Tuesday he does not yet have the votes to block new witnesses in the impeachment trial.
He convened a closed-door meeting of GOP senators shortly after Trump’s legal team made its closing arguments.
Matthew Knott is North America correspondent for The Sydney Morning Herald and The Age.