If President Obama — or his successor — replaces the late Justice Antonin Scalia with a strong liberal, the Supreme Court’s balance will swing dramatically to the left in the coming years. It might well be the biggest ideological swing in recent memory. Indeed, American politics has not seen a moment like this one since 1987. That was when the retirement of moderate Justice Lewis Powell, who was the pivotal vote on many big issues, left the court with four strong conservatives and four strong liberals. (The latter included Sandra Day O’Connor, whose votes through 1987 had been quite conservative.) […]