Political polarization between the two major parties runs deeper than divisions based on gender, age, race or class than at any time since Pew began tracking trends in American values in 1987, the survey specialist based in the nation's capital said.
Virtually all increases occurred while George W. Bush and Barack Obama occupied the White House -- with the bases of both Democrats and Republicans voicing criticism of their parties for not embracing their traditional positions.
Pew said 71 percent of Republicans and 58 percent of Democrats indicated they didn't think their parties have done a good job standing up for core beliefs.
For 25 years, Pew has conducted surveys assessing American values regularly, providing a series of historical benchmarks so any changes in what unites or divides people can be studied.
"Overall, there has been much more stability than change across the 48 political values measures that the Pew Research Center has tracked since 1987," the report said. "But the average partisan gap has nearly doubled over this 25-year period -- from 10 percent in 1987 to 18 percent in the new study."
Basic demographic divisions haven't expanded in recent years and "for the most part pale in comparison to the overwhelming partisan divide we see today," the report said.
The report looks to the make-up of the Democratic and Republican parties themselves for part of the explanation, noting that both have contracted and are more ideologically uniform. Self-described conservatives dominate the GOP while a small-but-growing number of Democrats say they are liberals.
Among Republicans, conservatives continue to outnumber moderates by about two-to-one. And there are now as many liberal Democrats as moderate Democrats.
But a declining number of voters identifying with party labels isn't the only reason.
"While many Americans have given up their party identification over the past 25 years and now call themselves independents, the polarization extends also to independents, most of whom lean toward a political party," the report said. "Even when the definition of the party bases is extended to include these leaning independents, the values gap has about doubled between 1987 and 2012."
Looking toward the 2012 election, the largest divides between ardent supporters of President Obama and presumptive GOP presidential nominee Mitt Romney turn more on the scope and role of government in the economics arena, Pew said, pinching the all-important swing voter.
"Their attitudes on the social safety net and immigration are somewhat closer to those of Romney supporters," the report said, "while they tilt closer to Obama supporters in opinions about labor unions and some social issues."
The latest values survey found little support for the idea of American "declinism," with a large majority of participants agreeing with the thought "as Americans we can always find a way to solve our problems and get what we want." Confidence in the nation hasn't waned even if Americans are more skeptical about economic growth prospects.
The latest Pew Research Center American Values survey polled 3,008 adults nationwide April 4-15. The values project has been updated 14 times since it began in 1987 and tracks a range of the public's fundamental beliefs. The margin of error 2.1 percentage points.
Andrew Kohut, who directed the study, told The Washington Post two things were notable.
One, "by and large, values haven't changed," Kohut said. "The other is that political identity has eclipsed ... other factors" such as race and class as the biggest sources of division.
"The only thing that's changed is the extent to which Republicans and Democrats go to opposite sides of the room on most issues," he said.
On some issues, such as whether the government should provide a social safety net, the gap already was wide and only has grown, Pew said. For other issues, such as religiosity and social conservatism, the originally small gap has expanded as well.
Republicans and Democrats are furthest apart in their opinions about the social safety net -- results indicate a gulf of at least 35 percentage points in opinions about the government's responsibility to care for the poor, whether the government should help more needy people if it means ringing up more debt and whether the government should guarantee all citizens enough to eat and a place to sleep.
On all three measures, the percentage of Republicans asserting a government responsibility to aid the poor has fallen in recent years to 25-year lows. Forty percent of Republicans agree "[it] is the responsibility of the government to take care of people who can't take care of themselves," a plunge from the 62 percent who expressed that view in 1987, near the end of President Ronald Reagan's second term.
Pew said less than half of Republicans (47 percent) agree "there needs to be stricter laws and regulations to protect the environment," a first in the political values survey. Eighty-six percent of Republicans held that view in 1992.
Democrats, too, have changed their opinions in the last 25 years, albeit less dramatically. They have become more secular, more positive in their views of immigrants and more supportive of policies meant to achieve equal opportunity, the survey indicated.
Demographically, Republicans remain overwhelmingly -- 87 percent -- white and, on average, are just less than 50 years old. Democrats, on the other hand, have become more diverse, with 55 percent being non-Hispanic whites, down from 64 percent in 2000.
Also, most Democrats are women and the average age of self-described Democrats is 47.7 years.
Independents also have become more diverse since 2000, with 67 percent of independents saying they are non-Hispanic whites, down 12 percentage points from 2000. The proportion of independents who are Hispanic has nearly doubled -- from 9 percent to 16 percent -- in the same period.