What the Senators and the country are entitled to know is how a judicial nominee regards his duty to respect the law as it is written -- especially the Constitution -- rather than vote according to his own "views," whether on abortion, religious symbols, or whatever.
Ideally, judges should respect both the Constitution and the legal precedents, for the same reason -- people rely on the law as it exists when they make decisions and commitments in their lives.
Telling people after the fact that the law is now different from what it was when they made their decisions creates problems for people who acted in good faith. Even a Justice who thought that the 1803 case of Marbury v. Madison was wrongly decided is unlikely to want to overturn two centuries of precedents based on it.
On the other hand, some of the precedents created by judicial activists more recently have gone so completely counter to the Constitution that it is a judgment call whether all of those precedents should continue to be followed. Judges take an oath to uphold the Constitution, not to regard all precedents as set in stone forever.
Respect for the separation of powers should apply to all three branches of government. Senators have no right to try to extort a pledge from a judicial nominee to vote a particular way on cases he has not heard -- and that is what Senators are doing when they talk piously about a "right to privacy" or other buzzwords.
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