There's one way of putting the Democratic argument about Roberts into perspective that I haven't heard yet.
Roberts continues to insist that he cannot answer any questions because of implications for his future position.
But his lack of respect for the powers of Congress, inherent in his failure to seriously respond to the questions of Senators, also has implications for the power of Congress. Democratic Senators must say that they cannot vote for Roberts. If they do, they risk encouraging witnesses that come before them not to respond to their questioning. They risk weakening the very branch of government they are charged to protect.