A cardinal fact of American politics that has emerged during the Obama years is that demographic forces are slowly and inexorably driving the electorate leftward. But the Republican Party has its own corresponding advantages. Its voters turn out for elections reliably, not just in spasms of quadrennial excitement. They are dispersed efficiently in rural and exurban House districts, and reside disproportionately in the states that have disproportionate representation in the Senate. All these things give the Republican coalition, even as it remains unable to muster a presidential majority, unassailable control of Congress.