John Hickenlooper, the Democratic frontrunner for the U.S. Senate seat held by Colorado Republican Cory Gardner, wasn't scheduled to attend a candidate forum Tuesday night in North Denver, and he won't be there Saturday for a debate in downtown Denver devoted to climate change, either.

The popular former two-term governor's habit of skipping more candidate forums than he's attended is drawing fresh criticism from event sponsors and his fellow primary candidates, whose name recognition and campaign bank accounts lag behind Hickenlooper's by significant margins.

"We already have a senator who avoids the public and dodges the press," said former House Speaker Andrew Romanoff, one of nine other Democrats running in the Senate primary, in a Tuesday fundraising email to supporters. "Let’s replace Cory Gardner with a leader who cares enough to show up."

Romanoff said that since Hickenlooper jumped from his White House bid to the Senate race in September he's skipped 15 forums sponsored by various groups, including veterans, students and climate activists.

Trish Zornio, a scientist and educator making her first run for office, similarly lambasted her primary opponent on Twitter on Feb. 8 after Hickenlooper "skipped yet another forum today, this time shafting a veterans group," adding, "If I wanted a Senator who didn’t show up, I’ve already got one in Cory Gardner."

Eight of the Democratic Senate candidates — not including Hickenlooper — have confirmed they'll attend a forum Tuesday night at Regis University sponsored by the school's student government and more than a dozen neighborhood and community organizations.

The forum, moderated by 9News political expert James Mejía, is set for 6:30-8 p.m. at the university's Mountain View Center at Claver Hall, 3333 Regis Boulevard, in Denver.

A Hickenlooper spokesman didn't respond to an inquiry about Tuesday night's forum but told Colorado Politics that the candidate declined an invitation to appear at Saturday's "Planet in Peril" debate because he had a "previously scheduled commitment elsewhere."

Hickenlooper also missed a climate-focused forum in September in Colorado Springs that was sponsored by many of the same environmental and progressive organizations behind Saturday's event.

The afternoon event on the Metro State University campus includes artists, musicians and a film screening, in addition to the debate and a separate panel discussion moderated by former state Rep. Joe Salazar, D-Thornton, executive director of the anti-fracking organization Colorado Rising and a persistent Hickenlooper critic.

The hour-long candidate debate is scheduled to start at 2 p.m. at the Tivoli Turnhalle, 900 Auraria Parkway, in Denver.

The other nine Democratic Senate candidates plan to be there, organizers said.

Tania Van Pelt, one of the organizers of Saturday’s event, said she was frustrated at Hickenlooper's refusal to attend.

"I understand he probably feels because he is the presumptive nominee, that it does him more harm than good to show up, given his record on climate," she said.

"But because he is the one anointed by the (Democratic Senatorial Campaign Committee), he could do us the courtesy and favor as voters and constituents to show up and tell us how he has evolved on the issue of climate, vs. being too scared or too entitled to show up. I find that really troubling."

"This isn’t some ‘gotcha’ set-up,” she added. "I would truly like to know why I should trust him."

Hickenlooper "believes climate change is both the biggest threat and the greatest opportunity of our time," campaign spokeswoman Melissa Miller told Colorado Politics.

His press secretary, Ammar Moussa, pushed back at suggestions Hickenlooper was ducking voters.

“John is making his case directly to voters in all of Colorado’s 64 counties about why he is the best candidate to beat Cory Gardner and change Washington," Moussa said in a statement.

"He has already participated in many forums across the state — including a climate change focused one in Denver — and will continue to talk with Coloradans about his plans to combat climate change, lower health care costs, and get Washington working for Colorado."

Senate candidate Diana Bray, a psychologist and climate activist, said she wasn't surprised Hickenlooper, a former petroleum geologist, "doesn't want to appear for a climate debate," citing "his extraordinary support of the fossil fuel industry" when he was governor.

Pointing to Hickenlooper's campaign contributions from oil and gas executives and his action as governor to block cities from banning fracking, Bray added: "People who are working to confront the climate emergency don't do these types of things, ever, and the only explanation for the former governor's lack of attendance is that he does not want his past actions to be exposed on a debate stage."

The other Democrats currently running for Gardner's seat include nonprofit director Lorena Garcia, women's and ethnic studies professor and former congressional candidate Stephany Rose Spaulding, immigration rights activist Michelle Warren, University of Denver professor David Goldfischer, former gubernatorial candidate Erik Underwood and author Christopher "Critter" Milton.