In this era of intense partisan polarization, it might seem odd—even quaint—to speak of a “center” of American politics. But a soon-to-be-released survey from the New Center, a bipartisan policy institute I helped start, suggests this term applies, albeit incompletely, to today’s American public. Both political parties ignore this center at their peril.

Let’s start with a definition. Suppose that for each issue there’s a continuum from one extreme to the other, defined by a scale from 1 to 5. There would be a meaningful center...