After giving an abbreviated defense on Saturday, Trump's lawyers were expected to shift the focus Monday toward Biden and Hunter Biden, who served on the board of a Ukrainian gas company. They've tried to portray the Bidens as corrupt in order to justify Trump's request for Ukraine to investigate them. The Bidens have denied any wrongdoing.
For his part, Starr argued that the country had foolishly entered "the age of impeachment."
"How did we get here?" he complained as he chronicled the history of the law and politics from President Richard Nixon's resignation in the face of threatened impeachment to Clinton's impeachment decades later.
"Like war, impeachment is hell. Or, at least, presidential impeachment is hell," Starr said. "Those of us who lived through the Clinton impeachment, including members of this body, full well understand that a presidential impeachment is tantamount to domestic war, but thankfully protected by our beloved First Amendment, a war of words and a war of ideas. But it's filled with acrimony and it divides the country like nothing else."
After defense arguments are finished, senators will get 16 hours to submit written questions followed by an opportunity to debate whether witnesses should be called. That debate could occur as soon as Thursday.
Bolton has offered to testify to the Senate if he is subpoenaed. He had resisted a request from the Democratic-controlled House during the impeachment proceedings there.
It's still unclear if four Republicans will join Senate Democrats to subpoena Bolton and possibly other witnesses.
Trump and Senate Majority Leader Mitch McConnell of Kentucky have sought to ensure a speedy path to acquittal without any witnesses.
Trump could try to block the testimony from his former national security adviser on the basis of executive privilege, which allows presidents to keep confidential their deliberative conversations with aides. But it's not clear that Trump has any enforcement provisions if Bolton decides to honor a Senate subpoena.
Democrats say a claim of executive privilege would fail because Bolton has already written his story in his book draft, which was submitted to the White House for classification review on Dec. 30 and is scheduled to be published in March, and because executive privilege is not a shield for presidential wrongdoing.