Democrats have said Trump used the aid to a vulnerable ally facing Russian aggression as leverage to get a foreign country to help him smear a domestic political rival.
Trump denied telling Bolton that he sought to use the aid to pressure Ukrainian President Volodymyr Zelenskiy to investigate the Bidens on unsubstantiated corruption allegations.
"I haven't seen the manuscript, but I can tell you nothing was ever said to John Bolton," Trump told reporters.
But moderate Republican Senator Susan Collins said the reports regarding Bolton's book "strengthen the case for witnesses."
"Every few days something comes out, and Americans are saying they want witnesses & documents," Senate Democratic leader Chuck Schumer said on Twitter
Starr, who himself recommended Clinton's impeachment after investigating the former president's sexual relationship with a White House intern, called impeachment an overused tool.
"Go to court. It really is as simple as that, I don't need to belabor the point," Starr said.
This is only the third presidential impeachment trial in U.S. history.
Another Clinton impeachment alumnus, Alan Dershowitz, carried a pair of worn antique books as he began closing the day's defense with an argument disputed by most legal scholars. "Purely non-criminal conduct, including abuse of power and obstruction of Congress, are outside the range of impeachable offenses," he said.
Dershowitz himself argued the opposite view during Clinton's impeachment.
