U.S. President Donald Trump's legal team has repeatedly made two main points for acquittal: the charges against Trump do not meet the constitution's criteria for impeachment. And if the president is impeached for abuse of power and obstruction of Congress, it will set a "dangerous" precedent.

Trump's lawyers maintain that the president was legally justified in freezing military assistance to Ukraine and that the case against Trump presented by Democrats in the Senate last week does not clear the bar of an impeachable offense.

Jay Sekulow, Trump's personal attorney, argued that the president "was at all times" acting under his constitutional and legal authority by withholding aid to Ukraine, saying that Trump's impeachment is about "deep policy concerns."

Meanwhile, pressuring is mounting for the U.S. Senate to subpoena testimony from former National Security Advisor John Bolton after a draft of his forthcoming book reportedly confirmed facts supporting the charges against U.S. President Donald Trump. 

According to an unpublished manuscript of Bolton's book, Trump told Bolton last August that Trump wanted to freeze 391 million dollars in military aid to Ukraine until its government agreed to investigate former Vice President, Joseph R. Biden, Jr. – a potential challenger for the U.S. presidency, and Biden's son Hunter.  

Bolton has been circulating drafts of his book to "close associates." He also sent a copy to the White House for review, which is standard practice. 

Trump denied Bolton's account: "I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens. In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book," he tweeted on Monday. 

Senate Democrats have been demanding sworn testimony from Bolton and other key witnesses. 

Stories about Bolton's account had increased pressure on a small group of Republicans to defy GOP leaders and join a bipartisan group of senators demanding more witnesses. 

"John Bolton's relevance to our decision has become increasingly clear," Sen. Mitt Romney (R-Utah) told reporters, while Sen. Susan Collins (R-Mass) has implied she is open to admitting new testimony. 

However, several GOP senators met privately with Senate Majority Leader Mitch McConnell and said in their opinion nothing had changed in the proceedings. 

Senate minority leader, Democrat Chuck Schumer, cited polls showing public support for witnesses. 

"We want the truth," Schumer said. "So do the American people."

Any vote on witnesses and new evidence would come after the defense team rests (it has up to 24 hours over three days) and after senators have a chance to ask questions in writing of both House prosecutors and Trump's legal team. The questions will be read out loud by Chief Justice John Roberts, who's presiding over the trial.