The issue of the day is "fair trial, and that requires witnesses."
Reframe the issue as those calling for more witnesses are undecided on removing Trump from office, that they are seriously considering the Senate should remove Trump from office. IOW, that there is a real case there, that what Trump thought or said (not what he did), is seriously grounds for removal from office.
McC might be worried that spite could impel senators who believe that a thought crime could be impeachable to vote to convict.