Seven of the 10 Democrats running for Colorado’s U.S. Senate seat plan to circulate petitions to land spots in the June primary — potentially setting up a crowded ballot for the chance to challenge U.S. Sen. Cory Gardner, the Republican incumbent.

Most of the candidates pursuing the nomination by petition say they’re relying on volunteers to gather the required 10,500 valid signatures — 1,500 from each of the state’s seven congressional districts — rather than planning to spend as much as the $450,000 strategists say a statewide petition drive could cost this year in Colorado.

The primary’s frontrunner, former Colorado Governor John Hickenlooper, and the newest candidate in the race, former gubernatorial candidate Erik Underwood, however, say they’re hiring petition circulators in addition to using volunteers to gather signatures.

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Petitions are due to the Colorado Secretary of State’s office by March 17.

The Democrats who have gotten the OK from state officials to start gathering signatures this week are Hickenlooper, nonprofit director Lorena Garcia, climate activist Diana Bray, immigration rights activist Michelle Ferrigno Warren and author Christopher “Critter” Milton, a candidate who has filed to run for the seat but hasn’t done much active campaigning in the year since the race began in earnest.

Underwood and another recent entrant to the primary, University of Denver professor David Goldfischer, haven’t yet had their petition formats approved by the secretary of state but told Colorado Politics that’s in the works.

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Garcia told Colorado Politics her petition drive is on track, with more than 20 trained circulators and more on the way. A campaign spokeswoman said Thursday that she’s already collected signatures in every congressional district.

Garcia said she’s “excited for the many volunteers who have committed to circulating petitions.” She added: “We are very confident that those choosing the caucus route will know that there is already a candidate on the ballot who has built a statewide grassroots coalition needed to win.”

Bray said she realizes a statewide petition is a heavy lift but is confident it’s something her campaign’s network can accomplish.

“I have an incredible team of volunteers,” she said. “I’ve been focusing for the last several months how to generate the team to do this.”

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Two of the petitioning Democrats — Hickenlooper and Underwood — say they’re also planning to go through the caucus and assembly process, which means there will be five candidates asking for support at the party’s March 7 precinct caucuses.

Candidates can qualify for the June 30 primary by winning the support of 30 percent of delegates at the Democrats’ state assembly, scheduled for April 18 this year at the Colorado Convention Center in Denver. The delegate selection process starts at caucuses.

The other candidates going through caucus are former state House Speaker Andrew Romanoff, women’s studies professor and former congressional candidate Stephany Rose Spaulding and scientist Trish Zornio.

Candidates aren’t bound to one method or the other and can switch from petitioning to the caucus route, or vice versa, as the campaign progresses.

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Bray, a first-time candidate, said she decided to go the petition route because she didn’t think caucusing made sense for “people who have never been in the political arena before.”

“It is something quite novel that we’re trying to do, but that’s what my whole campaign has been about. For an underdog, long-shot candidate, petitioning with volunteers is the only pathway,” she said.

“You don’t have to be a mathematician to realize that caucusing will only work for, at most, three people, and if one candidate gets 40% at assembly, then only two can make it onto the ballot,” Bray said.

“There are 10 of us running for the Democratic primary. Caucusing is not, for me, a pathway that I see would be a winning pathway.”