BASALT, Colo. — Claudia Cunningham had never voted for a Republican in her life. She swore she couldn’t or her father would roll over in his grave. But ahead of the Colorado primary on Tuesday, she did the once-unthinkable: registered as unaffiliated so that she could vote in the G.O.P. primary against her congresswoman, Lauren Boebert.
So did Ward Hauenstein, the mayor pro tem of Aspen; Sara Sanderman, a teacher from Glenwood Springs; Christopher Arndt, a writer and financier in Telluride; Gayle Frazzetta, a primary care doctor in Montrose; and Karen Zink, a nurse practitioner south of Durango.
Driven by fears of extremism and worries about what they see as an authoritarianism embodied in Ms. Boebert, thousands of Democrats in the sprawling third congressional district of Colorado have rushed to shore up her Republican challenger, State Senator Don Coram. Their aim is not to do what is best for Democrats but to do what they think is best for democracy.
It is a long shot: Mr. Coram has raised about $226,000 in a late-starting, largely invisible bid to oust a national figure who has raked in $5 million.
But as Mr. Arndt noted, anti-Trump Republicans have put aside stark differences with liberal policies and voted for Democrats since 2016. It is time, he said, that Democrats return the favor and put preservation of democracy above all other causes.
The Colorado crossover voters are part of a broader trend of Democrats intervening to try to beat back the extremes of the G.O.P., in Georgia, North Carolina, Colorado, Utah and elsewhere.
“The center has got to re-emerge,” said Tom Morrison, a lifelong Democrat in rural Pitkin County who voted for Mr. Coram, not only in protest of Ms. Boebert but also of what he calls a rising concern about his party’s leftward drift.
A nascent infrastructure is supporting the trend. The Country First Political Action Committee, established by Representative Adam Kinzinger, an anti-Trump Republican from Illinois, has used text messages and online advertising to rally opposition against what the congressman has called the most “toxic” and partisan Republicans. Those include Representatives Madison Cawthorn, Republican of North Carolina, and Jody Hice, Republican of Georgia, who, with Donald J. Trump’s backing, tried to defeat Georgia’s secretary of state, Brad Raffensperger, after he resisted Mr. Trump’s push to “find” the votes to nullify President Biden’s victory there.
In Utah, rather than backing a Democrat in a strongly Republican state, 57 percent of the delegates to the state’s Democratic convention, including Jenny Wilson, the Salt Lake City mayor and the state’s most powerful Democrat, endorsed Evan McMullin, a former C.I.A. officer and an anti-Trump Republican. He is running an uphill independent campaign against Senator Mike Lee, a Republican who initially worked to challenge Mr. Biden’s victory.
In Colorado, a constellation of small political groups have sprung up to oppose Ms. Boebert’s re-election ahead of next week’s primary, such as Rural Colorado United and the Better Than Boebert PAC, formed by Joel Dyar, a liberal community organizer in Grand Junction, and James Light, an affluent Republican developer who helped create the mega ski resort Snowmass in the 1970s.
“Jan. 6 was the breaking point for me,” Mr. Light said. “I couldn’t get anywhere with the national party, so I got behind Don Coram.”
Advocates for the strategy point to some success stories. In the Georgia secretary of state race, at least 67,000 people who voted in Georgia’s Democratic primary two years ago cast ballots in the Republican primary, an unusually high number. Mr. Raffensperger cleared the 50 percent threshold to avoid a runoff by just over 27,000 votes.
More than 5,400 early or absentee votes cast in the western North Carolina primary that included Mr. Cawthorn similarly came from Democrats who had voted in their party’s primary two years earlier. Mr. Cawthorn lost by fewer than 1,500.
In Colorado, voters can cast ballots in the Republican primary if they are registered with the party or as unaffiliated. In Ms. Boebert’s district, Democratic Party officials have tallied about 3,700 more unaffiliated voters in this year’s Republican primary compared with two years ago. They are largely concentrated in the Democratic hubs of Pitkin County, home of Aspen, where one can never be too rich or too liberal, and La Plata County, where Durango is filling with young people.
Mike Hudson, a Durango activist who worked for Democratic luminaries like Hillary Clinton and Marian Wright Edelman before “disaffiliating” in January to go to work for Mr. Coram, said the number of independents from both parties mobilizing against Ms. Boebert was “grossly underestimated.”
Ms. Boebert’s campaign did not respond to requests for comment. She remains a prohibitive favorite on Tuesday.
Almost no one would say that the influx of Democratic voters into Republican primaries this year has been driven by an organized effort.
“What did we do to reach out to Democrats? The answer is nothing,” said J.D. Key, Mr. Coram’s campaign manager. “This is completely organic.”
Some Democratic officials have tried to stem the effort, worried in part that Mr. Coram will be the more difficult Republican to beat in November, and in part that the newly disaffiliated might not come back. Dr. Frazzetta has emailed patients, left literature in her office, even pressed the compounding pharmacists she works with to consider voting in the Republican primary. Among the blizzard of positive responses was one harshly negative reaction, she said, from a local Democratic Party official.
A new map has made the district more Republican, but Mr. Trump won the old district with 52 percent of the vote in 2020, not a staggering total. Judy Wender, an Aspen Democrat who has resisted entreaties from friends to disaffiliate, said there was good reason to vote next week in the Democratic primary: Three very different Democrats will be on the ballot, and the right one could be a threat to Ms. Boebert in the fall.
Howard Wallach, a retired high school teacher from Brooklyn who runs the Pitkin County Democratic Party with his wife, Betty, was similarly disapproving. The Republican primary ballot includes several candidates from Ms. Boebert’s wing of the party, including a Senate candidate, State Senator Ron Hanks, who marched to the Capitol on Jan. 6; a secretary of state candidate, Tina Peters, who was indicted in March on 10 charges related to allegations that she tampered with election equipment after the 2020 election; and a candidate for governor, Greg Lopez, who has stood by Ms. Peters’s false election claims and said he would pardon her if elected.
Mr. Wallach asked: Will these voters new to Republican politics come prepared to choose in those races?
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“They’re desperate,” he said of the newly independent voters. “They’re crazed.”
Several Democrats said that attitude is part of the reason the nation finds itself at this crossroads, with two opposing camps, unwilling to find common ground in the center. The anger and fear stoked by Mr. Trump and his followers like Ms. Boebert may have “fertilized the ground for tyranny,” as Jackie Merrill, a newly disaffiliated Democrat, put it, but Democrats have played a part.
“Progressive Democrats keep believing that if they can just gain power, they can bring the country with them to all these liberal causes,” Mr. Morrison said. “And they can’t.”
In some sense, Ms. Boebert is a special case for the “disaffiliation” cause. Her 9,873-vote primary victory two years ago over a mainstream Republican, Scott Tipton, shocked voters here. If many Western Coloradans did not know the gun-toting restaurant owner then, they all do now.
She actively opposed the bipartisan infrastructure bill, though recently she has claimed credit for some of its projects, and on Wednesday, she led a group of House hard-liners denouncing the Senate’s compromise gun safety bill.
“She gets paid $174,000 a year so she can rage tweet,” Pete Tovorek, 52, said over lunch at the Miner’s Claim restaurant in Ms. Boebert’s hometown, Silt, Colo.
Above all, many Democrats and Republicans say, the vast district needs help, and Ms. Boebert shows no inclination to take her job seriously. The San Luis Valley in the south is parched by drought. The Colorado River is at a low ebb. Income inequality between Aspen and Telluride and the struggling areas nearby has exacerbated housing prices and labor shortages.
“We’re topsy-turvy,” said Mr. Hauenstein, the mayor of Aspen, where the median rental listing is $22,500 a month — “not a typo,” as The Colorado Sun put it.
Of course, Ms. Boebert has dedicated fans. Her biggest base of support is not on her home turf in Garfield County, in the western shadow of the high Rockies, but in Grand Junction. But Rob Baughman, of Meeker, Colo., in a county near Garfield, said he appreciated her uncompromising voice, even as his wife, Susan, decried her congresswoman’s lack of a “filter.”
In Ms. Boebert’s restaurant, Shooters Grill, on the picturesque main drag of Rifle, Colo., “Trump Won” and “Drill Baby Drill” T-shirts are for sale while waitresses serve food with handguns holstered on their hips. A patron called Ms. Boebert “a good answer to A.O.C.” (using shorthand for Representative Alexandria Ocasio-Cortez, a liberal Democrat) before a waitress ushered this reporter from the premise.
Regardless of the outcome, though, several Democrats said their decisions to vote in the Republican primary — and the pushback they received from old friends — had convinced them that the shape of political activism would have to change if a center was to re-emerge.
“All these dark mumblings about what would happen if you disaffiliate from the Democrats,” Ms. Cunningham said in amazement. “There’s a certain quality of disbelief in what’s happening among normal people. We have to get past that.”