Matthew Franck thinks appointing Clinton to SCOTUS would be a gift to Republicans:

Miller misses the most obvious blunder such a move would commit.  Promising the Democratic Party that he would appoint Hillary Clinton to the Supreme Court would put Barack Obama on record as committed to the proposition that the Court is just another political institution like any other.  Its vacancies would be viewed as political chips, to be wagered in a presidential campaign without regard to legal or constitutional categories of thought.