Chart shows exit poll data for voters supporting Republican congressional candidates since 1976; 2c x 4 inches; 96.3 mm x 101 mm; WASHINGTON (AP) - House Republicans feel growing pressure to steer firmly right on key issues, thanks to changes in primary-election politics that are complicating Congress' ability to solve big problems. Independent research supports the belief by these lawmakers that they owe their jobs to increasingly conservative activists, and that it's safer than ever to veer right on many subjects rather than seek compromise with Democrats. On the flip side, House Democrats face a more liberal-leaning electorate in their own primary elections. But the trend is less dramatic for Democrats, whose supporters are more open to compromise to help government work, polls show. And Republican control of the House makes the GOP dynamic more consequential. The House's recent struggles to handle once-routine tasks - such as passing a bipartisan farm bill and raising the federal debt limit - partly stem from the millions of Republican primary voters who elect representatives with stern warnings not to compromise with Democrats. It's also a reason that efforts to rewrite the nation's immigration laws face problems in the House, where Republicans quickly dismissed the Senate's bipartisan approach. In interviews, House Republicans often cite worries about a possible challenge from the right in their next primary. Many of them represent districts so strongly Republican that it's all but impossible for the party's nominee to lose a general election to a Democrat. Also, these lawmakers say, it's highly unlikely that a moderate Republican can wrest the party's nomination from a conservative incumbent. "There aren't a whole lot of moderate Republicans who participate in the primary in a conservative district," said Rep. Kenny Marchant, R-Texas. That leaves many House Republicans with only one prerequisite to assure their re-election: Never give a hard-charging conservative enough room on the right to mount a viable challenge in the primary. In practice, the task doesn't seem so hard. Only six House Republicans lost their re-election primaries last year. Half of them fell to fellow incumbents in redrawn districts that forced two colleagues to oppose each other. The other three lost to challengers with strong tea party support. Rep. Jean Schmidt's loss was instructive. A conservative by almost any measure, the three-term Ohioan was attacked nonetheless for voting to raise the federal debt ceiling and for giving President Barack Obama a peck on the cheek as he entered the House for his 2012 State of the Union address. Memories of what happened to Schmidt - and to veteran Republican senators such as Bob Bennett and Richard Lugar, who also lost primaries to tea party-backed challengers - come up repeatedly in political discussions, House insiders say. GOP lawmakers regularly take the temperature of their districts' conservative activists, who are crucial in primary elections, which often draw modest turnouts. "House members are better at reading their districts than anyone else," said Republican lobbyist and pollster Mike McKenna. McKenna said it's not unusual to discuss immigration reform with House Republicans who say, "I'm getting emails from people who vote in primaries. They say 'I don't care what the Farm Bureau says, I hate this stuff.' Rep. John Fleming, R-La., tracks such emails and phone calls. He said his office recently received 80 calls about immigration, "and all of them were against the Senate bill." The Senate bill would create a pathway to citizenship - or what many conservatives call "amnesty" - for millions of immigrants living here illegally. Fleming, asked whether he ever worries about going too far right for GOP primary voters in his district, said: "What's the chance of a moderate Republican coming in and saying, 'Oh, I'm for amnesty'?" Marchant, the Texas congressman, said he's a lifelong conservative who has watched GOP primary voters in his west-Dallas district lean increasingly to the right. Tea partyers who once cast their votes for Libertarian Party candidates, he said, now are full-fledged Republicans. "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. House Speaker John Boehner, R-Ohio, put a positive spin Sunday on what many see as congressional gridlock. "We should not be judged by how many new laws we create," Boehner told CBS's "Face the Nation." We ought to be judged on how many laws we repeal. Associated Press Director of Polling Jennifer Agiesta contributed to this report. Follow Charles Babington on Twitter: https://twitter.com/cbabington