The whole idea of giving Supreme Court justices lifetime tenure was so they wouldn't be beholden to political parties. If they always ruled along party lines so they would get re-elected or re-appointed, then they wouldn't be independent. If that was the case, what would be the point of having a Supreme Court? The constitutionality of laws would largely depend on the idealogy of whichever political party was more popular at any given moment. Whether I agree with a particular decision or not, I personally don't want a wishy-washy Supreme Court whose direction changes with every election.
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