BOULDER, Colo.—In the tiny storefront that houses the Boulder County Republicans, the air is thick with paranoia. At the front of the crowded room, the county GOP chairwoman, Ellyn Hilliard, is lecturing on voter fraud and the Democratic conspiracy she's convinced is afoot to steal the election. I had expected Boulder Republicans to be a slightly more conservative version of everyone else in this liberal college town—laid-back and crunchy and clad in polar fleece—but that, it seems, is not the case.

When Cory Gardner, the Republicans' much-hyped Senate nominee, bounds up onto a chair for a quick rev-'em-up speech, the contrast couldn't be clearer. "What can we do to make sure we are protecting this beautiful environment?" he asks. "What can we do to put more Colorado in Washington and less Washington in Colorado?" He talks about economic growth, making college affordable, and working across party lines. He talks about his grandparents and the American Dream.

Gardner, who is locked in a tight race with incumbent Mark Udall, has made it clear he's Not That Kind of Republican—not one of the cranky old men that have dominated the Colorado GOP for years, helping Democrats win by alienating the suburban swing voters who decide elections here. Indeed, Not That Kind of Republican might as well be Gardner's slogan—never mind that Democrats say it would be a lie.

A little fireplug of a man, moonfaced and brush-haired, the 40-year-old Gardner is so relentlessly upbeat it can be exhausting to spend time with him. He talks fast, in abbreviated clauses that don't always gel into complete sentences. His affect falls somewhere between a human ray of sunshine and an overcaffeinated hamster. When I interviewed him, he parried my questions with short, rapid-fire answers, anticipating each volley like a tennis player crouching behind the net.

Gardner, who is currently in the House of Representatives, readily admits his party has "overreached" in the past. He promises to be a pragmatist who focuses on kitchen-table issues. Democrats dismiss this as election-year posturing, but Gardner insists he's been consistent: As a state legislator, he notes, he advocated a renewable-energy bill over the Republican governor's veto; he has long been pushing his party for some sort of immigration reform (though he's slippery about what that involves); and last year, "a lot of nasty things were said about me in my own party about my unwillingness to demand the government shutdown," he says. Gardner acknowledges Obamacare isn't going anywhere as long as President Obama remains in office, and he says that if Republicans gain the Senate majority, "I'm going to be shouting from every desktop possible" in favor of an inclusive, bipartisan approach.

Gardner doesn't bring up any of the polarizing issues around which Udall has sought to frame the campaign, chiefly abortion. During this campaign, Gardner renounced his former support for the "personhood" initiative that will be on the Colorado ballot for the third time this year, saying he'd changed his mind after coming to understand the implications. But he remains a cosponsor of a federal personhood bill and has danced around aggressive questioning on the issue. (At one point, he even denied the federal personhood bill existed.) Frustratingly for Democrats, Gardner is a glib and talented politician, able to spit out non-answers with a cheery smile and without tripping up and saying something that could be used against him.

Democrats believe Gardner is just putting a happy face on the same old extremism. In 2012, they note, National Journal ranked Gardner the 10th most conservative member of the House. (In 2013, he was 98th.) Despite his talk about immigration reform in the abstract, they point out, he's voted to rescind DACA, Obama's executive action sparing some young illegal immigrants from deportation, and he doesn't support a path to citizenship. (When I pressed him on this, he said he was for "some kind of earned status," but as for citizenship, "I don't know that that is a universal demand by anybody.") While he claims he opposed the shutdown, he wasn't outspoken about ending it.

"The tactic of Democrats in Colorado has been to create a narrative about the Republican candidate six months before the election"—that they're in the pocket of Big Oil or a social-issues-obsessed ideologue—"and then opportunistically wait for the Republican to fulfill it," says Rob Witwer, a former GOP state legislator who's been close to Gardner since they came to the statehouse together. Past candidates like Tom Tancredo and Ken Buck always fit the bill. But "the narrative they created for Cory Gardner doesn't fit him. He hasn't behaved the way they said he would," Witwer says. "He's not the person they said he would be, and now, as voters see he's not that kind of person, it's become a credibility issue for Udall."

Witwer has spent much of the last decade advocating for a kinder, gentler, more unified GOP. He co-wrote a book, The Blueprint, detailing how Democrats took over Colorado through a combination of well-financed, coordinated advocacy and Republican infighting. (A more sinister take on the plotting cabal of rich, secretive liberals can be found in a new Citizens United film, Rocky Mountain Heist, narrated by Michelle Malkin and released this month.) After the 2012 election, when Obama won Colorado for the second time, Witwer wrote an op-ed for the Denver Post titled, "Republicans must improve or die."

To Republicans like Witwer and Ryan Call, the moderate Colorado GOP chairman, Gardner is a godsend: a principled conservative who doesn't come across as an impeachment-obsessed crank. Colorado Republicans are still dysfunctional—several moderates were defeated by fringe candidates in legislative primaries this year—but they've unified to a remarkable degree around Gardner, perhaps as much out of desperation as anything else. On the other hand, if he can't win, many fear the state may be lost to the GOP for good.

The comeback of Not That Kind of Republican is what even many Democrats say they want, decrying the GOP's rightward march even as they mine it for electoral advantage. On Monday night in Aurora, a diverse South Denver suburb, Democratic activists filled a high-school gymnasium to hear former President Bill Clinton pine for the sane Republicans of yesteryear. Gardner's ticket-mate, gubernatorial nominee Bob Beauprez, once questioned Obama's birth certificate and supported repealing the 17th Amendment, which mandates direct election of senators. "When he won the primary, people said he was a moderate, and I said, our standards are getting a little loose!" Clinton said. "It doesn't take much to qualify as a moderate Republican these days!"

Beauprez, like Gardner, has been able to position himself as moderate in part because the party has moved even further right. But Gardner has also pulled a neat trick, one that Clinton—the original New Democrat—might recognize: He's taken the Democrats' own strategy and turned it against them. It was Clinton, after all, who once turned his party around by running as Not That Kind of Democrat—a centrist who could break with the ideological left and focus on pocketbook issues. A decade later, Colorado Democrats also began to succeed by being Not That Kind of Democrat—focusing on quality-of-life issues like education, health care, and the environment, while Republicans descended into angry, gun-toting paranoia. They ran, essentially, as moderate Republicans.

Just look at Gardner's opponent, Mark Udall. Tall and craggy, with a slightly aloof demeanor, he's a skilled mountain climber who looks less like a senator than the head of Outward Bound—which he was for a decade before entering politics. Posed on a ridgeline in a ski jacket, Udall is a veritable mascot for the Colorado culture—in Clinton's words, "He looks like an ad for Colorado." When he first ran for office, he seemed authentic and nonpolitical.

But now it's Udall who's been cast in the old Republican role: angry and obsessed with social issues, a partisan tied to an unpopular president. Gardner calls him a "social-issues warrior" who "has nothing else to talk about." In a clever move, after coming out against personhood, Gardner came out in favor of making birth-control pills available over the counter. Opponents say this would make them less accessible to low-income women, but it's a position that makes it hard for Democrats to assert that he's against contraception—or that he doesn't have a positive agenda. Other Republican candidates have since copied the position.

After Gardner finished speaking in Boulder, a protest broke out outside. Half a dozen Hispanic activists waved signs and chanted, "He's trying to take my DACA back! Gardner is wack!" Conveniently, one of the Boulder Republicans was prepared for just such an occasion. He went to his truck and brought out his own pre-made sign reading, "DETECT - DETAIN - DEPORT." One of Gardner's staffers confided to an onlooker, "I don't know why they're protesting. Cory's one of the Republicans who supports what they want." Gardner, meanwhile, stayed inside, shaking hands and chatting. When he signs a bumper sticker, he jokes, "I heard you get 5 percent more fuel efficiency when you put this on!"

Gardner holds a narrow lead in the polls, though Democrats have reason to believe the polls are wrong: In 2010, Senator Michael Bennet (the brother of The Atlantic’s editor in chief and co-president, James Bennet) defied the polls to win. But even if Gardner prevails, they note, he will have done so on a platform of birth control, college loans, and environmental protection. "If Gardner wins, it will be because he ceded the argument," says Laura Chapin, a Denver-based Democratic consultant. "He's campaigned as a Democrat on reproductive rights and immigration!"

If Udall loses, however, the co-opting of their message will be cold comfort to Colorado Democrats.