(CBS News) -- When it comes to political ideology, Americans are more divided than at any point in the last 25 years, according to a new survey from the Pew Research Center for the People and the Press.
The poll shows Republicans and Democrats are increasingly far apart on questions surrounding values and beliefs. In 1987, when Pew began gathering data on how members of both parties feel about a set of 48 value-oriented questions, the average disparity between Republicans and Democrats was 10 points. In 2012, the average was an 18-point difference.
Much of the increase has been relatively recent: Between 1987 and 2002, the differentials fell within a 3-point range, alternating between 9, 10, and 11 points. In 2003, however, the average difference in partisan voter values jumped from 11 to 14 percent, where it remained constant until 2007. In 2009, the number jumped to 16 percent, and in 2012 it reached 18 percent.
While partisan polarization has surged in recent years, the differentials have remained relatively stable among other demographic groups, including race, income, education, religiosity, and gender.
"We've been asking the same questions in the same way for 25 years," said Carroll Doherty, one of the associate directors of the poll, in an interview with Hotsheet. "The partisan divide is now by far the largest single gap among the public and the parties are more polarized than they ever have been in that 25 year period."
According to the survey, conducted from April 4-15 among 3,008 adults nationwide, both Democrats and Republicans have become more ideologically homogeneous in recent years. A majority of Republicans call themselves conservatives, while an increasing number of Democrats identify as liberal. Conservative Republicans outweigh moderates by two to one, and there are about as many self-proclaimed liberal Democrats as moderate Democrats.
The most pronounced divides are between committed supporters of President Obama and Mitt Romney on questions about the scope and performance of the government, as well as issues related to the economy, environment and immigration. Between 1987 and 2012, the partisan division on the role and scope of the government jumped from 6 to 33. Between 1992, when the question was first asked, and 2012, the differential between Republicans and Democrats on questions surrounding the environment catapulted from 5 to 39. On immigration, the difference has jumped from 4 in 2002, when Pew started asking the question, to 24 in 2012.
"The supporters or Barack Obama and Mitt Romney are very far apart on nearly all issues, but what's a little less clear is where roughly a quarter of the voters come down." Those so-called swing voters, Doherty says, are "cross-pressured" -- meaning that "on some issues they side with Romney voters and on some issues they side with Obama voters."
The poll also notes a growing number of Americans who choose to identify as independents rather than Democrats or Republicans.
Thirty-eight percent of the survey's respondents said they identify as independents, while just 32 percent of called themselves Democrats, and 24 percent identified as Republicans.
"The number of independents continues to rise," said Doherty. "It's at the highest percentage in more than 70 years, and the percent affiliating with a party is at its lowest percentage."
Doherty cited a growing dissatisfaction with both parties -- as well as Washington -- as part of the reason for that change.
"Roughly 10 years ago you had roughly equal numbers of Republicans, Democrats and independents," Doherty said. "But as we've seen the parties pull apart, the number of independents has grown."
Lucy Madison, CBS NEWS