America, it seems, is stuck—unable to make significant progress on critical issues such as climate change, rising economic inequality, and immigration. To explain that inaction, people often point to political polarization. Democrats and Republicans, liberals and conservatives, are now so sharply opposed to each other that they are unable to find common ground.

But while the country is stuck, it is not stationary. Some things are changing; it’s just not at the federal level that the changes are emerging.

Polarization leads to stalemate only under certain circumstances—when the two sides in a conflict are closely balanced, and political institutions and procedures (such as the Senate filibuster) enable each side to check the other. That has been the story in the federal government.

Under other circumstances, however, polarization can be a stimulus to change. When politics become polarized between two alternatives, voters have clearer choices. They have more reason to pay attention and turn out. Each side may then mobilize, take power, and get its way in different jurisdictions or private institutions. That is what is happening now in state and local governments and civil society. Two ideologically based societies have developed within the United States, and the differences between them are growing.The question will ultimately be which America, red or blue, dominates the nation’s future...