This piece by Jason Brennan argues that everyone's theory of what's "constitutional" is pretty much whatever they want to have be "constitutional." Key quote:
It seems that almost everybody does the following:
1. Start with a political philosophy – a view of what you want the government to be able to do and what you want to the government to be forbidden from doing.
2. Take the Constitution as a given.
3. Reverse engineer a theory of constitutional interpretation such that it turns out – happily! – that the Constitution forbids what you want it to forbid and allows what you want it to allow.
I think Mr. Brennan overstates his case. I know lotsa Liberals who believe gun ownership should be seriously restricted but concede that the Second Amendment says what it says. I know lotsa Conservatives who believe pornography should be seriously restricted but concede that the First Amendment says what it says.
On the other hand, one could argue that those examples are also cases of conceding lost causes. Liberals know they're never going to get a serious restriction on gun ownership and Conservatives know they're never going to rid the land of porn. What Brennan says is kinda true for matters where people think they have a fighting chance.