Republican U.S. Sen. Cory Gardner on Wednesday said he has accepted invitations to five fall debates and issued a challenge to the Democrat who wins the U.S. Senate nomination to join him.

Colorado voters are set to start receiving mail ballots for the June 30 primary in three weeks. Former Gov. John Hickenlooper and former state House Speaker Andrew Romanoff are vying for the chance to run against Gardner, who is seen as among the most vulnerable Republican senators on the ballot this year.

“At such a crucial time in our state’s and nation’s history, Coloradans must have an opportunity to hear directly from those who wish to represent them as they make their choice for U.S. Senate,” Gardner said in a statement.

“Debate is a pillar of our democracy – especially during a time of national crisis – and I look forward to discussing the issues Coloradans care most about as we approach November and look towards the future of our country.”

The Gardner campaign said the incumbent has agreed to participate in debates sponsored by these media and civic organizations:

? KOAA News5, Colorado Politics, The Gazette and El Pomar Foundation’s Forum for Civic Advancement on Sept. 1 in Colorado Springs

? Club 20, a Western Slope advocacy organization, on Sept. 19 in Grand Junction

? KDVR, KXRM and KFQX on Sept. 29 in Denver, also airing in Colorado Springs and Grand Junction

? KCNC, The Colorado Sun and PBS 12, which used to be called Colorado Public Television, on Oct. 14 in Denver

? The Pueblo Chieftain, on a date to be determined, in Pueblo

Gardner’s spokesman got in a jab at Hickenlooper, who has skipped more than a dozen candidate forums since the beginning of the year.

“With one candidate citing scheduling conflicts and lack of sleep for ducking debates throughout the Democratic primary, we thought it would be best to give both Andrew Romanoff and John Hickenlooper as much notice as possible for the fall debate season,” said Jerrod Dobkin, the Gardner campaign’s communications director. “We are hopeful our eventual opponent will show up to these debates that voters across Colorado are anxiously awaiting.”

Hickenlooper and Romanoff took part in a virtual forum last week that included a prohibition on the candidates asking each other any questions. A month ago, after he secured a spot on the primary ballot, Romanoff challenged Hickenlooper to seven debates, but so far none have been scheduled, spokespeople for both campaigns confirmed.

The Gardner campaign said it won’t consider additional debate invitations until both Democrats have responded to the five debate invitations it has accepted.

Notably absent from the list of debate sponsors: KUSA, the Denver TV station whose 9News news department – a news partner of Colorado Politics – has sponsored general election debates between Colorado’s U.S. Senate candidates every cycle for the last two decades. A spokesman for Gardner told Colorado Politics the campaign on Wednesday declined a debate invitation from KUSA and a consortium of media outlets from the northern Front Range and the southwest corner of the state.

Gardner and then-U.S. Sen. Mark Udall participated in five general election debates in 2014 in the two months before Gardner unseated the Democrat.