SCOTUS has no police or armies to enforce its edicts. The institution functions entirely on "trust capital." Every time it strikes down some democratically-instituted law, it uses up some of that trust capital. Sometimes, it must spend that capital. But doing it too often erodes faith in the institution, and ultimately, that is the only thing they have.
I'd argue that Citizens United is a great example of that in action. In my opinion, the Court got the law absolutely right in that case. But they used up a lot of trust capital in the process.
SCOTUS has far more than faith. It derives its powers from the constitution, and a constitutional amendment would be needed to destroy it, not merely a lack of faith, and until its destroyed it has the ability to set precedent and make rulings. To say it has no police or armies is simplistic as well, once departmentalization of constitutional interpretation starts occurring, you have a divided nation on the path to civil war. The thought that there wouldn't be Americans willing to take up arms in solidarity with supreme court rulings, and with SCOTUS rulings as their justification is rather naive.
The only time the executive has ignored supreme court ruling I believe was when Lincoln was in charge, and well, to say the nation was divided at that time would be an understatement.
I'd argue that Citizens United is a great example of that in action. In my opinion, the Court got the law absolutely right in that case. But they used up a lot of trust capital in the process.