They vote along lines of political philosophy based on knowledge of the Constitution. ‘Parties’ change with the wind whenever it’s expedient.
The American political fault line is how much or how little centralized national government should exist. It doesn’t matter what the political parties call themselves it always comes down to that. As far as the Rat appointed USSC justices go all of them view the states as quaint relics of the past. If they had their way the states would be nothing more then lines on a map. Any other view would interfere with the central government’s ability to build utopia.