This year’s Republican primary ballot in Colorado’s 4th Congressional District could be the longest of any congressional primary in state history.

As many as nine GOP candidates could make the June 25 primary for the seat held for five terms by Ken Buck, whose resignation from Congress takes effect on March 22.

Buck’s announcement last fall that he wouldn’t seek reelection drew nearly a dozen Republicans hoping to replace him in the most reliably Republican congressional district in a state, where the GOP has seen its once-dominant standing steadily decline over the past two decades.

Crowded primaries are nothing new, but the race to succeed Buck, a former chairman of the state Republican Party, will likely go down as the most surprise-filled electoral contest Colorado has ever seen.

Two months after Buck announced his planned retirement from Congress, U.S. Rep. Lauren Boebert, the flamboyant Western Slope Republican who narrowly won reelection in 2022, set state politics on its ear when she announced she was moving from the competitive district she’s represented for two terms into Buck’s district, where Boebert will have a much better chance of winning if she can get past the primary.

Rather than clear the field, the incumbent’s district switch — derided as “carpetbagging” by multiple opponents — appeared to energize her primary rivals, many of whom can trace their roots in Eastern Colorado back for generations.

There was another stunning twist around the corner.

On March 12, Buck did something no other U.S. House member from Colorado had ever done before when he announced he would resign his seat at the end of the following week, setting in motion a special election to fill the remainder of his term.

Because Buck set his resignation date for March 22, under Colorado law the election could only take place on either the Tuesday a week before the primary or on the June 25 primary itself, so Gov. Jared Polis picked the latter. Holding the elections concurrently simplifies things for county clerks while potentially complicating things for candidates and voters, who will be marking the ballot twice for the same office — once to pick Buck’s temporary replacement and a second time to nominate candidates for the November election.

Republicans plan to designate a nominee for the special election on March 28 at a convention in Hugo — near the geographic center of the 4th CD, Tom Wiens, a former state senator and the district’s GOP chairman, told Colorado Politics — and Democrats are slated to pick their nominee in a virtual convention on April 1.

As for the primary, it could take until late April before that line-up is finalized.

Colorado law offers major party candidates three options to get on the primary ballot: going through their party’s caucus and assembly process, petitioning, or taking both routes simultaneously.

The assembly track requires winning support at their assembly from at least 30% of the delegates, whose paths to county, district and state assemblies begin at precinct caucuses. This year, Democrats and Republicans held caucuses during the first full week of March, following the state’s presidential primary, with a series of assemblies scheduled into mid-April.

The GOP’s 4th CD assembly is set for April 5 in Pueblo, the day before the Republican state assembly, and 4th CD Democrats plan to convene virtually on April 11, with their state assembly also happening online, on April 13.

The candidate who comes out ahead at assembly secures top-line designation on the primary ballot, with any additional candidates’ positions determined in a random drawing.

Candidates can also win a spot on the ballot by gathering the required number of petition signatures from fellow party members who live within the jurisdiction of the office they’re seeking. Petition circulators have roughly two months to collect signatures, starting in mid-January, with this year’s deadline to submit petitions of March 19.

For Colorado’s major party congressional candidates, it takes 1,500 valid signatures to qualify for the primary. That compares to 12,000 signatures needed for candidates for governor and the U.S. Senate — 1,500 from each of the state’s eight congressional districts — and 8,000 for other statewide executive offices, or 1,000 from each district, though none of those offices are on the ballot this year. Legislative candidates need 1,000 signatures, whether they’re running for the state House or state Senate. Rarely, candidates can qualify if they gather 30% of the number of votes casts in the previous cycle’s primary for the same office, if the total is less than the stated numerical threshold.

Independent and minor-party candidates for the same offices have different petition requirements and deadlines, as do other offices whose petitions are also verified by the Colorado Secretary of State’s Office, including RTD directors. Candidates petitioning at the county level — including commissioner, sheriff and assessor candidates — submit their petitions to their county clerks.

Candidates who take the hybrid assembly-petition route must meet the same signature requirements but only have to win support from at least 10% of assembly delegates to make the primary.

This method used to work differently when Colorado’s primary elections were scheduled later in the year — decades ago, the state’s primary was held in September — and petition deadlines fell weeks after the last assemblies, so candidates who fell short of making the ballot by assembly but still cleared 10% could, theoretically, still mount a petition drive.

These days, with petitions due before most assemblies take place, the option is usually chosen for strategic reasons, like to keep potential rivals from the ballot, or to demonstrate strength among party regulars.

New this cycle, state Republicans have changed party bylaws to allow the party to take sides in primaries against candidates who only petition, providing another incentive for candidates to seek the approval of delegates, even if it means risking their position on the ballot if fail to reach 10%.

In the 4th CD this year, an unprecedented eight Republican candidates turned in petitions by the March 19 deadline, with all but two of them coming in just under the wire.

The order candidate turn in their petitions matters, since voters’ signatures are only counted for one candidate in each race, no matter how many petitions for a particular office they signed. If a voter signs more than one petition for the same office, it doesn’t matter which one they signed first, because petitions are verified in the order they’re submitted.

That sets up a sort of game of chicken among petitioning candidates in crowded fields — trying to make sure they have enough signatures but also trying to beat their opponents to the punch, so anyone who signed their petition won’t be disallowed because those voters’ signatures were counted first on a rival’s petition.

Boebert was the first of the district’s Republicans to turn in petitions, near the end of February, and, on March 11, she learned that she had easily qualified for the primary, with a little over 1,000 more valid signatures than she needed.

Douglas County nonprofit head Deborah Flora — who barely missed making the 2022 U.S. Senate primary at the GOP state assembly — got hers in next, on March 8, and could learn soon whether her petition is sufficient.

The other six Republicans running for Buck’s seat turned in their petitions in the final two days before the deadline, on March 18 and 19.

Those candidates, in the order their petitions were stamped in, are: state Rep. Mike Lynch, Logan County Commissioner Jerry Sonnenberg, state Rep. Richard Holtorf, oil and gas advocate Floyd Trujillo, former congressional staffer Chris Phelen and business consultant Peter Yu, who was the 2018 Republican nominee in the 2nd Congressional District.

The Republicans at the front of the line have a better chance that too many of their signatures won’t be knocked out by candidates whose petitions are verified before theirs — a strong possibility, since candidates tell Colorado Politics that competing petition circulators were lined up outside prime spots in the district, such as supermarkets and big box stores.

Still, it’s possible all eight of the petitioners managed to gather enough unique signatures — with more than 188,000 Republicans in the 4th CD, there are more than enough to go around — that they all gain berths in the primary. The Secretary of State’s Office has until April 26 to process all the submitted petitions, which could go faster than in recent cycles, since there aren’t any statewide races with their massive numbers of signatures to check.

A ninth Republican, former state Sen. Ted Harvey, is the only primary candidate running in the district who swore off petitioning entirely and is instead attempting to get on the ballot solely through the assembly.

While it’s mathematically possible for as many as three candidates to make it out of assembly with 30% or more of the delegate vote, it’s more likely that only one or two emerges, especially if candidates whose petitions have yet to be verified roll the dice on the alternate route, splitting the vote.

Ernest Luning has covered politics for Colorado Politics and its predecessor publication, The Colorado Statesman, since 2009. He’s analyzed the exploits, foibles and history of state campaigns and politicians since 2018 in the weekly Trail Mix column.

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