Former Gov. John Hickenlooper, one of nine Democrats in the running to challenge Sen. Cory Gardner in 2020, won't be attending a climate change forum for candidates on Oct. 6.

A campaign spokesperson confirmed the news Sunday night, saying the Democrat couldn't make the event in Colorado Springs because of a "scheduling conflict."

All of the other major Democratic candidates plan to attend — as well as two Unity Party candidates. Republican Cory Gardner did not respond to an invitation according to organizers.

"John Hickenlooper believes climate change is both the biggest threat and the greatest opportunity of our time," Hickenlooper for Colorado spokeswoman Melissa Miller explained in an emailed statement. "While a scheduling conflict, unfortunately, prevents him from attending next weekend's forum, he looks forward to talking with voters across the state about his plans to combat climate change by growing our clean energy economy and reinstating and expanding protections for clean air and water."

Miller declined to clarify the nature of the conflict.

Climate change has become a defining issue of the Colorado Senate primary. Other Democrats have attempted to use the topic to distinguish themselves from Hickenlooper, who has a long record of trying to balance environmental priorities with oil and gas development. The geologist-turned-politician famously once told the Senate he drank hydraulic fracturing fluid to prove the substance was safe. As governor, he also threatened to sue any local government that tried to ban fracking in 2013.

As a presidential candidate, Hickenlooper repeatedly criticized aspects of the Green New Deal as too expensive and ambitious.

Senate candidate Andrew Romanoff, a former Democratic state Speaker of the House who many see as Hickenlooper's chief rival, has embraced the Green New Deal. He's also signed a pledge not to take any money from fossil fuel companies for his campaign. The candidate noted both points in a tweet Sunday and said he was "disappointed" to hear Hickenlooper wouldn't join the climate forum.