It added that Mr Trump had offered to appear at the debate on condition Fox News contribute US$5 million (S$7 million) to his charities, which it refused to do. The figure fell off sharply from December's CNN debate, which had more than 18 million viewers.

Mr Trump's no-show plunged the presidential race into uncharted waters just days before Iowans vote on Feb 1, opening the months-long nomination process to pick the Democratic and Republican candidates for the November election.

During his speech on Friday, Trump lamented that Cruz was not questioned during about his birthplace during Thursday's Fox News debate.

As for the debate, Trump predicted it would have far fewer viewers without him on the stage.

Whether it was strategically beneficial for Trump to hold a rival event. Trump's rivals, Cruz, Rubio, former Florida Gov. Jeb Bush and New Jersey Gov. Chris Christie, all planned a full day of campaigning in the state.

Cruz, as the second-highest polling candidate in Iowa after Trump, predictably bore most of the attacks on the Des Moines debate stage with the fiercest exchanges coming between him and Florida senator Marco Rubio as they battle for conservatives and evangelicals in Iowa.

If Trump wins the Iowa caucuses, the move will be lauded as proof that he has built a movement capable of circumventing establishment media. Trump is fond of the term, though liberals have criticized him heavily for using it.

According to CNN, Mr Trump and a small number of legal scholars have claimed the Constitution's "natural-born citizen" requirement necessitates the President must be born on U.S. soil.

Bush defended rounds of anti-Rubio attack ads.

This week, the answer was F. Let us explain. "Well, that creates an environment that's toxic in our own country", Bush said.

When asked about his opinion on Iowa, 2016 Donald said, "I love Iowa, we've done really well here", whereas 2015's Trump asks, "How stupid are the people of Iowa?" "I was treated very badly and very unfairly by Fox". He even chided two lesser-known presidential candidates, former Maryland governor Martin O'Malley and former Virginia governor Jim Gilmore.

"They called me just before you walked on the plane", he said. Trump told the crowd at Drake University on Thursday night, where he announced that he has raised $6 million that will be donated to veterans through his personal foundation.