Beware the purists. To get anything done in this messy, contentious world, you have to accept that other people have different values and objectives than you do. The trick is to find some overlapping middle ground where you get part of what you want. In a divided democracy of 330 million people, compromise is essential; as Washington politicians once implicitly understood, half a loaf is better than none. But in recent years, our politics have become deeply polarized, as the bases of the Republican and Democratic parties have moved further right and left, respectively. Cutting deals has become a lost, or perhaps abandoned, art. That's why the government briefly shut down last weekend. It's why forging a compromise on immigration that can pass both houses of Congress — and get past presidential adviser Stephen Miller's veto — will be so difficult. Both parties' bases are demanding total victory, and calling congressional leaders who are willing to take half a loaf wimps and sellouts.

Most Americans, I believe, still fall somewhere within a few degrees of center on the political spectrum, and instinctively distrust extremists. But in an overwrought political climate defined by the shouting partisans on cable news, moderation and compromise have fallen into ill repute. That's a guarantee of gridlock and dysfunction. In a defense of moderation, conservative writer Peter Wehner has pointed out that a smart strategist can be driven by moral ideals even while charting a pragmatic course. "Moderation," he says, "takes into account what is needed at any given moment; it allows circumstances to determine action in the way that weather patterns dictate which route a ship will follow." Victories achieved without some buy-in from the opposition are often short-lived — and followed by intense backlashes that can wipe out nearly everything that was gained. On immigration, as on many issues, there is a reasonable middle ground — if the purists will let us get there.