For purposes of analysis of what a senate trial will be like involves 3 groups of Republican Senators.
The first group is very small and may only be Susan Collins. She is hurt by Trump regardless. That is Trump being at the head of the 2020 ticket makes it less likely she will win reelection. In her case, and maybe a few others, the political expedient action may actually be to vote for removal or at least insist on a real trial.
The second group, consisting of Mitt Romney, Lisa Murkowski and about 20 others, do not need Trump to hold office but need Trump to remain in the majority. That is without Trump’s base of racists and others who vote against their own best economic and other interests and created group 3, these senators would remain in the senate but in the minority and have diminished or no real power. But with Trump acting if not becoming a dictator, they must ask, what real power they have with Trump.
The third, largest and most diverse group, includes everyone from Cory Gardner to Lindsey Graham and Mitch McConnell who need Trump’s base of racists and their ilk to even hold office. That is several, like Gardner may lose anyway but without Trump have no chance of holding office.
In my view the 2nd group is the group to watch, those that will hold office regardless of Trump and his base. Collins and others who might be helped in their elections by voting to impeach Trump could still lose regardless. As Paul Krugman pointed out today, the reason ALL Republican, including all those leaving in the House are sticking with Trump over impeachment is they all want employment after leaving and between FOX and other extremist Rightwing “think tanks” that actually pay more and require much less effort than elected office, so long as they stick with Trump they will almost certainly get an easy life after leaving office.
But those 20 senators who are not looking for a golden parachute and in seats they will likely hold for as long as they want, have little personally to gain from choosing the constitution over Trump. If the Republican dam or damned, is to break, it will be from these senators.