Nolan McCarty, professor of politics and public affairs at Princeton University, has examined the voting records of every member of Congress since the 1800s. He created a “Polarization Index’’ that reflects how often members of the two parties agreed on major issues. His index shows that the parties agree less now on the issues than they did in the era after the Civil War.

“You can go down the list of things where there was not a single Republican vote,’’ McCarty said. “Even after the Civil War, there were some bipartisan issues, like tariffs and inflationary monetary policy.’’

McCarty says one reason for the change is rising income inequality. During times of relative equality — the 1940s through the 1960s — politicians played to the middle class, regardless of their party, and had significant overlap in their ideas. But starting in the 1970s, the nation began to divide much more starkly into rich and poor.

The Democratic Party catered more to lower-income people and minorities who wanted more government services, even if that meant higher taxes. Meanwhile, the Republican Party catered more to wealthier people, including a newly emerging class of entrepreneurs and business owners, who wanted less government intervention and lower taxes, even if it meant fewer services, he said.

But others see different reasons for the rift and are more optimistic about overcoming it.

Senator Ron Wyden, a liberal Democrat from Oregon who won reelection Tuesday after touting his bipartisan efforts, blamed special interests for pushing lawmakers into extreme positions that average Americans don’t want.

“There is no question that there are much more powerful forces that get up every single morning and say to themselves, ‘We want to deep-six these efforts to find common ground,’ ’’ he said. “But I think the hand of those who want to find common ground is potentially stronger now, because it is not going to be possible for either side to go to voters and say, ‘We achieved our objective by stopping everything.’ ’’

Wyden sees fruitful areas of cooperation, citing a tax-reform bill that he has worked on with Judd Gregg, the retiring New Hampshire Republican senator, that would reduce the corporate tax rate but also take away incentives for sending jobs overseas. He also said the two parties could work together to speed up federal waivers that allow states to create more choice for people buying health insurance under the new health care law.

Jim Matheson, Democrat of Utah, a leader of the Blue Dogs in the House, said the two sides should work on creating jobs. He said he believes polarization has been spurred by the fact that small number of ideologues in both parties control the nomination process.

Some studies suggest that, as the two parties pull farther apart, more Americans are identifying as independents, feeling that neither party truly represents them. During the 2008 and 2010 elections, about 37 percent of American voters described themselves as independents, Keeter said.

Mark Baldassare, research director of the Public Policy Institute of California, said the shifting preferences of those swing voters help explain why Democrats won in 2008 but Republicans won Tuesday.

Senator Scott Brown, the Massachusetts Republican who was elected in January, is viewed as the first beneficiary of the Tea Party movement but took a number of moderate positions this year, including support for a financial-regulation bill. He told reporters Thursday that he had voted with Democrats nearly one-third of the time but “they’ve voted with me zero. So it’s a two-way street.’’

Many see the partisan tilt in this election as bad news for Senators Olympia Snowe and Susan Collins, two moderate Republicans from Maine, a state that elected a more conservative governor and Legislature.

In a statement to the Globe, Snowe, who is up for reelection in 2012, said that “Congress must focus like a laser’’ on tax and regulatory policies that would foster economic growth and spur job creation, starting with the expiration of the Bush tax cuts, which will raise taxes across the board in January. She did not address the issue of bipartisan efforts.

But Collins, who is up for reelection in 2014, pledged to keep working with all parties, including newly elected moderate Republicans.

“The solution is more balance in Washington,’’ she said.

Farah Stockman can be reached at fstockman@globe.com