Just compare the records over the last three decades. Democrats have appointed four justices — Ruth Bader Ginsberg, Stephen G. Breyer, Elena Kagan and Sonia Sotomayor. All have been consistent liberals on the bench. Republicans, by contrast, have picked seven justices. Of Ronald Reagan’s three appointees (Sandra Day O’Connor, Antonin Scalia and Anthony M. Kennedy) only Scalia has been a consistent conservative. George H.W. Bush appointed one solid conservative (Clarence Thomas) and one disastrous liberal (David Souter). With George W. Bush’s appointments of Samuel A. Alito Jr. and Roberts, conservatives thought finally they had broken the mold and put two rock-ribbed conservatives on the bench — until last week, that is, when Roberts broke with the conservatives and cast the deciding vote to uphold the largest expansion of federal power in decades.

So Democrats are four-for-four — a perfect record. Republicans are not even batting .500.

Why is the Democratic record so consistent while the Republican record is so mixed? For one thing, the whole legal and political culture pushes the court to the left. Conservatives are pariahs if they vote against the left on certain issues. But if they cross over vote with the left, they are hailed as statesmen. Just look the pre-emptive attacks on the Roberts Court when everyone thought it was about to strike down Obamacare — and contrast that with all the accolades Roberts is now receiving from his erstwhile critics.