They only need two-thirds of the senators present. So if the Republicans absent themselves, they could get 100% of those present to vote "guilty." That would obviously be a travesty but they saw nothing wrong with ramming through the second impeachment in seven hours without Trump being permitted to make a defense.
Lewis Carroll, you should be living at this hour.
If all the Republican Senators were refusing to be seated, the democrats would play hell explaining what they were doing and proving in the Supreme Court that it was Constitutional. All they would have there then would be Mitt Romney with all the democrats.