In the world that I argued for, liberal senators would support Kavanaugh. They would do so because he is qualified, because the decision to give President Trump the authority to select judges was made when the electorate chose Trump over Hillary Clinton, and because the cost of opposing Kavanaugh—that Republican senators would oppose similarly qualified judges when the presidency is in Democratic hands—exceeds the possible benefits of opposition. This set of assumptions was driven by norms: not by law, not by the Constitution, not even by Senate rules. It was just the way things worked, because everything worked better if everyone behaved that way. And everyone behaved that way because everyone had confidence that those on the other side would behave that way as well.
Polarization put cracks in that confidence. And once people no longer believed the other side would observe the norm, the norm collapsed remarkably quickly.
Today, I have no good answer to a liberal who says he can’t support Kavanaugh because he lacks confidence that Kavanaugh will affirm Roe v. Wade. Why should he support a justice who won’t deliver the political results he wants? Similarly, I had no good answer to Republican senators who wanted to hold Antonin Scalia’s seat open to see what happened in the election. Why shouldn’t they have supported Mitch McConnell in freezing out Merrick Garland?