"The mainstream Republicans, as a result, have become more conservative," Marchant said. Tea party activists, he said, "found that they could go into the Republican primary and make a real difference."
GOP Rep. Howard Coble, elected to 15 terms from central North Carolina, dates the change in primary voters' behavior to the mid-1990s. Conservative groups, he said, "were challenging Bob Dole for not being pure enough."
"That has opened the gates to primary races" against Republican incumbents, Coble said.
Voter surveys support the view that Republican voters are becoming more conservative.
On average, from 1976 through 1990, 47 percent of people who voted Republican in House races considered themselves conservative, according to exit polls. A slightly smaller share called themselves political moderates.
During Bill Clinton's presidency — which included bruising fights over health care, gun control, taxes and his impeachment — Republicans' conservatism began rising. From 1992 through 2006, GOP voters were 52 percent conservative on average and 41 percent moderate.
And in the most recent House elections, 2008 through 2012, more than 6 in 10 voters who backed a GOP candidate described themselves as conservative. About a third called themselves moderate.
Meanwhile, those who vote for House Democrats have become more liberal. But self-described liberals still comprise less than half of that group. In the pre-Clinton years, 25 percent considered themselves liberal; 33 percent on average did so from 1992 to 2006; and it stands at 40 percent across the last three elections.
Michael Dimock, who tracks such trends for the Pew Research Center, said that several years ago there was a notable difference between social conservatives and business conservatives in the Republican Party. Today, he said, Republican voters are more unified — and solidly conservative.
"The socially conservative right has adopted that anti-government, small-government principle, and it's largely consolidated," Dimock said.
Rep. David Price has watched the two congressional parties grow farther apart for decades, first as a Duke University political science professor, and for 25 years as a Democratic House member from North Carolina.
At a recent Yale University conference on Congress, Price said: "Reaching agreement was extraordinarily difficult in the 1990s. It seems almost impossible now."
Associated Press Director of Polling Jennifer Agiesta contributed to this report.
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