It isn't just that Democrats and Republicans agree on less and less these days. It's that they hate each other's guts.

In Congress and in state houses across the country, differences of opinion are widening when it comes to taxation, social issues and more. Beyond any specific disagreement about a particular bill or budget, though, negotiation has become trickier as legislators seek compromises with people whom their constituents simply detest.

Ordinary Americans increasingly view members of the opposite party with contempt and scorn. They see them as less intelligent and more selfish, according to pollsters and political scientists. And parents are more likely to say they wouldn't even want someone in the other party to marry their children.

America dodged a catastrophe last week as Congress voted to extend the national borrowing limit for two more years — just another day on the job for our increasingly divided lawmakers. The next crisis isn't far off, though. In a governmental system of checks and balances, just keeping the lights on requires cooperation from everyone, warns Alan Abramowitz, a political scientist at Emory University.

American politics has entered a "fundamentally different era," he says.

Abramowitz is one of several scholars who have been trying to solve a puzzle in polarization. Americans are becoming more likely to vote consistently for one party or the other, but for some reason, they aren't more likely to describe themselves as committed partisans.

The apparent solution is that even though voters aren't any more dedicated to their own parties, they have a newfound antipathy for the other party, and they are less willing to consider casting a ballot for one of the other party's candidates.

As shown in the chart, the average voter's feelings about his or her own party haven't changed much over the past few decades, according to the American National Election Studies, a recurring survey. Respondents are asked to rate their own party and the opposing party out of 100 points, with larger numbers indicating more positive feelings. They have consistently given their own party a rating of between 70 and 80.

Meanwhile, that average voter's views of the other party have become more negative, according to the surveys. And more voters said they felt anger and fear toward the other party's presidential candidate in the 2012 election than in any previous survey. Roughly two-thirds of voters said the other other party's candidate made them angry, and nearly half felt afraid.

That distaste extends beyond presidential candidates in the other party to their supporters, according to various surveys compiled by Stanford University political scientist Shanto Iyengar and his colleagues in an alarming paper that cites Hunter S. Thompson's "Fear and Loathing on the Campaign Trail."

In 1960, for example, surveyors asked Americans to rate members of their own party and of the other party for intelligence and selfishness. Respondents saw members of their own party as slightly more intelligent, by about 6 percent of the points on the scale, and somewhat less selfish, by about 17 percent of the available points. Those differences expanded to 48 percent and 43 percent, respectively, in another survey conducted in 2008.

Americans have also become less likely to see members of the other party as a suitable daughter- or son-in-law. Back in 1960, only about 5 percent of Republicans and 4 percent of Democrats told pollsters they would be "displeased" if their child married someone from the opposite political party. By 2012, 49 percent of Republicans and 33 percent of Democrats said they would be at least "somewhat unhappy" if one of their children had a bipartisan wedding.

'A deepening reservoir'

Americans' views of the major parties have changed at least in part because the parties themselves have changed. For one thing, it used to be that you couldn't always tell much about a person's political views based on his party, because there were both liberal and conservative voters in each party. Today, by contrast, party is a reliable index of political inclination.

In places like West Virginia and the Deep South, there are still conservative voters who cast their ballots for Democrats, but there are fewer and fewer of them. If you really dislike people with liberal opinions, it makes sense to denigrate Democrats as a group now, whereas before, there might have been enough conservative Democrats to redeem the party from your point of view.

"Those value disagreements used to divide people within the parties on both sides 30 or 40 years ago, now they divide Republicans from Democrats," said David Kimball, a political scientist at the University of Missouri at St. Louis. He is one of the authors of another recent paper that alludes to Thompson's work, which discusses "a deepening reservoir of fear and loathing" in American politics.

That reservoir is available to the leaders of the parties, who are themselves drifting apart and can rely on their constituents to support them as they do. Ideological ratings of votes in Congress and in state legislatures show that fewer and fewer centrist lawmakers hold office in both parties. Republicans in particular now subscribe to more extreme views.

Moderate candidates are less likely to win nominations when primary voters have similar views. Those candidates then use more extreme rhetoric that arouses voters' antipathies toward the other party on both sides. "Partisans are more willing to believe whatever awful things their side says about the other side," Kimball said.

Abramowitz called it a "feedback loop."

Politics is national

Scholars debate whether polarization among politicians or among the public is the more important factor, and whether hardening views among the public have more to do with substantive questions of policy or with less tangible differences, such as their mutual perceptions of intelligence and generosity.

What does seem clear, though, is that American politics has changed. Campaigns are now less about your guy than they are about the other guy. How enthusiastic a candidate's supporters are about voting for him or her matters less than how angry and scared they feel about the prospect of the opponent winning.

And since partisans' disdain for the other party isn't directed at particular people and politicians who make it up, but toward the group as a whole, individual differences between candidates will probably matter less in future campaigns, Abramowitz argues.

These days, voters are more likely to cast a ballot of their party based on their perceptions of the other party. Those perceptions depend on the other party's national leaders, Abramowitz says, and the statements they make in the national media.

"All politics is local" is an old saw in American politics often attributed to Rep. Tip O'Neill (D-Mass.), the former speaker of the House. Abramowitz contends that the maxim is out of date.