I doubt that very few of either party are going to vote on principles, not theirs (they have few) or the Constitution’s.
Expediency.
Theyre gonna vote the way their constituents leans. Safe Blue districts will vote yes. Safe Red districts will vote no.
Swing districts could go either way.
The Constitution was designed to put personal expediency to use for the common good. The Founders knew damn well that if they had to depend on principled men, the country would turn into a banana dictatorship before they could even invent that cliche. Men who knew what was good for them were easier to find, and still are. So long as they don’t get excessively greedy or power hungry, that will suffice.
So how should a Republican vote if he voted on principles? Yes???
Senator or Member of Congress: "How much bribe money and how many votes are in it for me? Okay. I'll vote on principles this time."