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DES MOINES, Iowa — The super PACs supporting Ted Cruz are offering $1.5 million to veterans charities if Donald Trump agrees to debate Cruz one-on-one before the Iowa caucus, heightening the stakes around Cruz’s proposal just five days before the Iowa caucus.

"Not only would this be a heck of a debate, but it would also be a terrific opportunity to generate millions of dollars for the veterans," the donors backing the pro-Cruz "Keep the Promise" entities said in a joint statement on Wednesday night.

Cruz laid out a specific proposal to Trump at a rally in West Des Moines on Wednesday evening, telling the crowd his campaign has reserved a venue in Sioux City, Iowa, for 8 p.m. local time on Saturday night. He said 800 seats are available for Iowans — and Trump can choose to do what he likes with half of them.

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“We have the venue. We have the time,” Cruz told the receptive, chuckling crowd. "All we’re missing is the candidate."

Trump has a rally scheduled Saturday night in Davenport, Iowa, which is on the eastern border of the state. Sioux City, where Cruz is proposing, is on the western edge of the state.

Earlier on Wednesday, Trump tweeted his response to Cruz’s offer: “Even though I beat him in the first six debates, especially the last one, Ted Cruz wants to debate me again. Can we do it in Canada?"

Cruz told NBC News on Wednesday night that candidates have to “show the respect and the humility to submit yourself to the voters and answer their questions."

Meanwhile, candidate Carly Fiorina looked to up the ante late Wednesday night, offering to donate $1.5 million to veterans causes to debate Trump and Cruz on Saturday night or $2 million to debate just Trump on Thursday night at Drake University here in Des Moines.

Cruz first chided Trump after his rival bowed out of Thursday night’s GOP debate in Des Moines. Trump is opposed to Fox News’ decision to use host Megyn Kelly, who he has called not “professional at all,” after testy exchanges between the two in the first Republican debate in August.

"I promise you Putin is a lot scarier than Megyn Kelly," Cruz said on Tuesday. "I'd like to hear Donald explain to the American people and to the people of Iowa how he is prepared to be commander-in-chief if he's terrified by a television host.”

The super PACs said in their statement that the debate must last an hour and the pair may choose the moderators to their liking.

Cruz has already proposed that radio and TV hosts Mark Levin, Sean Hannity or Rush Limbaugh as moderators, or open the floor to the audience to ask questions.