Even today, you have a fair amount of Republicans who criticize Trump for various reasons, including Mitt Romney, the 2012 nominee, and John Kasich. Additionally the new "Lincoln Project" is funded and run by anti-Trump Republicans.
This is actually exactly what I'm talking about, not the proceedings themselves. With the exception of the potential wildcard of the Bolton testimony (which never ended up happening) at that point the party had essentially been whipped and it was just theatre.
Here are the names that were discussed incessantly in conservative media: https://www.vox.com/2019/10/14/20908684/senate-republicans-t...
Up until Matt Gaetz did his stunt disrupting the private/secret hearings of House witnesses there wasn't very much fire on the Republican side which was why these close-door pre-impeachment vote hearings happened in the first place. Remember that at certain point there was a pseudo-impeachment process occurring prior to any impeachment vote having taken place. Even convincing Republicans that they needed to fight to get an actual vote on the record was not an simple process.
https://www.vox.com/policy-and-politics/2019/10/23/20929023/...
"During an interview with Sean Hannity on Fox News, McConnell said that "everything" he does "during this, I'm coordinating with the White House counsel. There will be no difference between the president's position and our position as to how to handle this, to the extent that we can."" (https://www.courier-journal.com/story/news/politics/mitch-mc...)
"But it evidently has great value to the president and to Mr. McConnell, who had spent nearly a year preparing for it. From the instant that Democrats assumed power in the House last January, denying that they had any intention of impeaching Mr. Trump, Mr. McConnell, a six-term Kentuckian and the longest-serving Senate Republican leader, directed his staff to quietly dig into the history of impeachments and consult with outside experts.
"“We thought they would finally work themselves up to doing this on something,” Mr. McConnell said. “It has been threatened endlessly. We needed to come up to speed on what actually happens, and that began in earnest last fall.”
"So when Mr. McConnell fielded a phone call from Mr. Trump days before Christmas, he was ready. Stung by the House vote to impeach him on two charges, the president reached out to the majority leader from his Mar-a-Lago retreat in Palm Beach, Fla., throwing out ideas about how to handle his coming Senate trial.
"Mr. McConnell had a reassuring response for the third president ever to face removal by the Senate, urging Mr. Trump to trust him to manage the confrontation." (https://www.nytimes.com/2020/02/06/us/trump-impeachment.html)