Today's political polarization is more than a journalistic trope. It is more intense than at any time in the past century, and it pervades our political system from top to bottom. It feeds legislative gridlock and damages trust and confidence in political institutions. Abroad as well as at home, observers question America's ability to govern itself as the times require.

This condition did not develop overnight. Half a century ago, the two parties agreed on Cold War anticommunism as the core of foreign policy and on a broadly Keynesian approach to economics. Most of the cultural issues that dominate today's landscape and divide the parties were not matters of public conversation.

By 1980 this postwar consensus had collapsed. Democratic support for Cold War anticommunism waned in reaction to the Vietnam War. Republican support for Keynesian economics had given way to the supply-side revolution. The country had split over new social issues—notably feminism, abortion and the counterculture. And the civil-rights movement triggered political realignments in the South and among the white working class in the Northeast and Midwest.

All these shifts pointed in the same direction, toward increased unity within each political party and more-intense divisions between them. Today, ideology, policy preferences, partisanship and voting behavior are aligned as never before.

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According to Alan Abramowitz, a political scientist at Emory University, party loyalty has reached the highest level in the history of survey research, and ticket-splitting the lowest. Of self-identified Democrats, 93% voted for President Obama in 2012; 93% of Republicans voted for Mitt Romney, and about nine in 10 voters supported members of the same party in the presidential, House and Senate elections. More than seven in 10 self-styled independents lean toward a political party and vote in patterns almost indistinguishable from more forthright partisans.

This partisan unity has assumed geographical dimensions. In the presidential contest of 1976, according to Mr. Abramowitz, 20 states with 299 electoral votes were decided by margins of 5% or less. By 2012, only four states with 75 electoral votes were that competitive. Other research has documented the rising number of state governments controlled by a single party, and even the rising number of supermajority counties.

Which party bears more responsibility for this state of affairs? In Congress, the answer is clear: While Democrats have moved toward the left, Republicans have moved much further right.

Among voters, the picture is more complex. In 1972, for example, 29% of Democrats called themselves liberal or very liberal, a figure that rose by 18 points to 47% by 2012. During those four decades, the share of Republicans regarding themselves as conservative or very conservative rose by fully 30 points, to 76% from 46%. The average Democratic voter moved about half a point to the left on the standard seven-point ideological scale; the average Republican voter moved about three-quarters of a point to the right. Republicans are now a predominantly conservative party, while Democrats remain a coalition of liberals and moderates.

On the role of government, Republicans have moved much further right than Democrats have to the left. It is hard to overstate the intensity of Republican sentiment on this issue. Between 1972 and 2012, the share of Republicans who regard themselves as very conservative on taxes and using government to promote jobs and social services soared to 38% from 9%, and the share of economic conservatives overall rose to 80% from 48%.

But on social and cultural issues, the picture looks different. In 2012, according to Mr. Abramowitz, "Democratic voters were somewhat farther to the left of center than Republican voters were to the right of center."

A recent Gallup survey bears him out. Between 1996 and 2014, the share of Americans supporting same-sex marriage doubled to 55% from 27%. Among Democrats, support rose to 74% from 33%—an astounding 41 points. But it rose among Republicans as well, to 30% from 16%. The parties are further apart on this issue, not because Republicans became more conservative but because Democrats moved more to the left.

This is consistent with another Gallup finding: Since 2000, the share of Democrats who regard themselves as liberal has risen more than the share of Republicans who call themselves conservative. In 2000, 44% of Democrats identified as moderate, 29% as liberal. Today, fully 43% call themselves liberal and only 36% moderate. At the same time, conservatives rose to 70% from 62% of GOP identifiers.

Amid this welter of statistics, one thing is clear: The U.S. government has become dysfunctional, and there is a shared responsibility to fix it. Leaders must behave differently, which will not happen unless the people insist on a different kind of governance. We can light a candle or curse the darkness.