Republican presidential candidate Carly Fiorina has offered to donate $2 million to veterans' causes if Republican front-runner Donald Trump agrees to a one-on-one debate Thursday evening in Iowa after the real estate tycoon pulled out of the GOP sanctioned debate that night. 

"My campaign will also donate $1.5 million to veterans' cause to join debate Saturday night in Sioux City," Fiorina tweeted late Wednesday, referring to a challenge from Sen. Ted Cruz (R-Texas) that would put him on a stage with Donald Trump.

Fiorina followed up by "upping the ante," suggesting she would join Trump at his Thursday night event at Drake University with a $2 million check in hand for veterans' causes. 

"Upping the ante, I'm free tomorrow at 8pm, @realDonaldTrump. My campaign will give $2mm to veterans' cause to debate at Drake U," she wrote.

Earlier in the day, Fiorina attacked Trump as an "ultimate insider," USA Today reported"He has made billions of dollars buying people inside of the political system," she said. "He freely admits he is a crony capitalist, folks. He's not going to challenge the system."

Fiorina's offer to Trump came after Ted Cruz formally challenged the New York billionaire businessman in a letter to debate in Sioux City, Iowa Saturday night. "We owe it to the men and women of Iowa to ensure that they hear jointly and directly from the two leading Republican candidates so that they may contrast our positions on the critical issues we face as a nation as they make their final choice leading up to Monday's caucuses," Cruz wrote in a letter to Trump, the Sioux City Journal reported

Attempts to get Donald Trump on stage followed his announcement that he would boycott Thursday night's GOP debate in Iowa after an ongoing set of disagreements with Fox News executives and host Megyn Kelly reached a head earlier this week. 

"What's interesting here is Trump is not used to not controlling things as the chief executive of a large organization," Kelly said on Tuesday night, according to RealClear Politics. "But the truth is, he doesn't get to control the media."