WASHINGTON – Sitting at his desk on the Senate floor Thursday evening, Sen. Lamar Alexander took a break from his note-taking and flipped through a book he brought with himas both sides made their case during President Donald Trump’s impeachment trial.

He leaned back and scanned the pages of “Impeachment: An American History,” in which four experts chronicled previous times U.S. presidents have faced impeachment.

Alexander had a front-row seat to the current proceedings, listening intently to the arguments made over dozens of hours and several weeks. But the clock was ticking and he was ready to address the issue that had unexpectedly thrust the 79-year-old retiring senator from Tennesseeinto the center of the trial: whether to support Democrats’ call for witnesses, notably Trump’s former national security adviser John Bolton, which most of his GOP colleagues in the Republican-controlled Senate opposed.

Having had the history of impeachment at his fingertips, Alexander said the lack of bipartisan support in the Democratic-led House impeachment process against Trump “struck” him. Had even a few House Republicans voted in favor, he might have viewed the case differently.