Rep. Paul Cook served 26 years as a Marine and was awarded two Purple Heart medals for combat wounds suffered in Vietnam. But amid his seventh year in Congress, the aching and discouraged California Republican has decided he's endured enough. At 76 and nursing brittle knees that make cross-country flights an ordeal, Cook is past the age when many lawmakers head home. Cook, who announced last week that he won't seek reelection next year, is the oldest of 18 House Republicans who have said they are leaving. He also is a case study in how some old-school moderate Republicans — the type that like him have Chamber of Commerce backgrounds and consider compromise laudable — feel alienated in an increasingly fractious capital. Cook believes Republicans will not win back the House majority in the 2020 elections, leaving the GOP once again with frustratingly little clout after spending most of this decade controlling the chamber. He dislikes President Donald Trump's late-night tweets and his criticisms of NATO and some of its member nations. And he bemoans Washington's 'toxic' political atmosphere, which he blames on hard-right Republicans and hard-left Democrats. 'The Freedom Caucus, a lot of them, they have a very right-wing agenda that encourages the same things that the far left does. And that is, 'We're going to raise hell,'' Cook said in an interview. 'That is not the road that I would advocate if you're going to try and reach a compromise. Our whole government is built on compromise.' His reference was to the House Freedom Caucus, about 30 fervent conservatives who try pushing the GOP rightward. 'We're mean. I don't know how else to say it,' Cook said. Conservatives say they make no apologies for not yielding on what they consider core principles such as the right to possess guns. 'I think that sometimes the fights are worth having and I'm happy to be in them,' said one Freedom Caucus member, Rep. Matt Gaetz, R-Fla. Of the 18 exiting Republicans, only a handful have openly cited Washington's sharp elbows, though others who've retired previously have expressed similar complaints. Three of this year's other departing lawmakers could have faced tough reelection fights. Three are running for the Senate or governor, two are leaving as their allotted time as committee leaders is expiring and others have voiced variants of, 'It's time to go.' 'Most moderates are fighting through it. We're not a dying breed,' said Sarah Chamberlain, who heads the moderate Republican Main Street Partnership. Nonetheless, lawmakers concede that frequent, consuming conflicts between the parties take a toll. Both Democrats and Republicans have fewer House moderates than they did two decades ago. That's partly due to increasingly sophisticated district lines that strongly favor one side or the other, putting a premium on candidates voicing ardent adherence to party dogma. As a result, it's been harder for each side to find supporters from across the aisle for legislation. 'I don't know if frustration is the right word. There's an acknowledgement that it's become a zero-sum game on both sides of the aisle,' said Rep. John Katko, R-N.Y., a leader of the Tuesday Group of moderate Republicans, which claims around 50 members. GOP leaders reject the assumption that they won't recapture the House majority, following last fall's drubbing in which they lost 41 seats. Rep. Tom Emmer, R-Minn., who leads House Republican's political committee, says that while 2018's Democratic challengers 'didn't stand for anything,' next year they'll have to defend hot-button progressive proposals like the Green New Deal. Cook is from safe, conservative GOP terrain: a vast, dry district that sprawls along the Nevada border and includes the Mojave Desert and Death Valley. Trump took it easily in 2016 and Cook has never won less than 57 percent of the vote since his first election in 2012. He hopes to win local office when he returns. Cook worked for his area Chamber of Commerce, taught history at colleges and was elected to local government and the California Legislature before coming to Congress. His legislative focus has included military and veterans' issues, and he's had a low public profile. He's tweeted 15 times all year on his official Twitter account — fewer than Trump might unleash on a raucous day. But Cook says he's bothered by a White House that takes positions that contradict prior policy, particularly on foreign affairs. He recoiled at Trump's questioning of NATO's value and episodes such as his musings about purchasing Greenland, a territory of Denmark that the NATO member spurned. 'You have to respect your allies or you're not going to have your allies when you need them the most,' Cook said. Cook tempers his criticisms of Trump, who endorsed him last year when he faced a conservative GOP challenger. Like most Republicans he's stood by Trump on most House votes, backing him 97 percent of the time, according to the nonpartisan website Fivethirtyeight.com. Rep. Jared Huffman, D-Calif., a friend since they served together in the California Legislature, calls Cook 'a prince of a human being.' He says he thinks the ex-Marine privately disdains Trump, citing Cook's military service and sense of honor. Cook is 'a prisoner to a Republican politics that has turned into a Trump cult,' Huffman says. Most of all, Cook laments a hardball political atmosphere that he says inhibits compromise. 'I have a lot of great colleagues here,' he said. 'It's just a very difficult environment when you should be striving to see the other point of view.