A five-way race in the GOP primary in Congressional District 5 will have a four-way debate Thursday night at Woodland Park High School.

The name not on the roster is perhaps the best known: incumbent Doug Lamborn.

Why? It’s not clear. Lamborn’s camp “declined to participate,” according to Erik Stone, chairman of the Teller County Republicans.

The debate is the only one scheduled before the June 26 primary. It’s being put on by the school’s speech and debate team.

Stone said he didn’t know and couldn’t speculate as to why Lamborn decided to skip it.

Lamborn’s spokesman, Dan Bayens, couldn’t be reached by phone and didn’t respond to a text message from Colorado Politics. The U.S. House calendar lists this as a “district work week,” and lawmakers don’t reconvene in Washington until next Tuesday. Lamborn’s official schedule lists no events this week.

His campaign website mentions only a fundraiser Monday at the at the El Paso County Republican Headquarters.

We’ll update this story if we hear back as to why Lamborn is skipping what could be his only chance to face off against his primary challengers.

Candidates who agreed to debate are state Sen. Owen Hill, El Paso County Commissioner Darryl Glenn, former Green Mountain Falls Mayor Tyler Stevens and former Texas judge Bill Rhea.

Lamborn has struggled in his bid to win a seventh term. The race has drawn a large field of well-known primary challengers. Then Lamborn had to go to court to get on the ballot after the Secretary of State’s Office said he didn’t have enough qualified petitions.

Nonetheless, a poll last week suggested Lamborn had a 10-point lead over second-place Darryl Glenn. But 21 percent of those polled said they were still undecided.

“This year’s primary is unprecedented for Colorado because for the first time ever, unaffiliated voters will be able to vote in the primaries,” Stone said about the debate. “This is the only opportunity for Republican and unaffiliated voters to see the challengers side-by-side in a debate before the primary ballots go out in early June.”

A reception begins at 6:15 p.m. followed by the debate at 7 p.m.

Colorado Politics and the Colorado Springs Gazette plan to cover it.