https://www.nationalreview.com/2020/01/how-mcconnell-outplayed-pelosi/
The Republican leader unified his caucus by relying on precedent.
Mitch McConnell was clear when he addressed the Senate on December 18: Any impeachment trial of President Trump would follow the precedent established by the trial of President Clinton 20 years ago.
Clinton’s trial was divided into pieces. The Senate agreed unanimously to begin with a briefing, opening arguments, questions from senators, and a vote to dismiss. Whether to hear witnesses or introduce additional evidence were questions decided later. “That was the unanimous bipartisan precedent from 1999,” McConnell said. “Put first things first, lay the bipartisan groundwork, and leave mid-trial questions to the middle of the trial.”
The arrangement satisfied Chuck Schumer back when he was a recently elected junior senator from New York. Funny how times change. Now Senate minority leader, and looking to damage Republicans in a presidential election year, Schumer demanded that McConnell call witnesses and ask for additional documents at the outset of the proceedings. Pelosi followed his cues. After the House impeached Trump on December 18, she said she wouldn’t transmit the articles of impeachment until McConnell gave in to Schumer’s demands.