In a way, politics as a whole now resembles the longstanding politics of abortion — a subject that puts fundamental values at odds, in which each side believes that the other side is morally in the wrong. Almost 38 years have passed since Roe v. Wade, and this dispute is no closer to resolution.

Yet we have, for the most part, managed to agree on certain ground rules in the abortion controversy: it’s acceptable to express your opinion and to criticize the other side, but it’s not acceptable either to engage in violence or to encourage others to do so…

Right now, each side in that debate passionately believes that the other side is wrong. And it’s all right for them to say that. What’s not acceptable is the kind of violence and eliminationist rhetoric encouraging violence that has become all too common these past two years.

It’s not enough to appeal to the better angels of our nature. We need to have leaders of both parties — or Mr. Obama alone if necessary — declare that both violence and any language hinting at the acceptability of violence are out of bounds. We all want reconciliation, but the road to that goal begins with an agreement that our differences will be settled by the rule of law.