In a rare show of solidarity, Donald Trump's Republican presidential rivals Mike Huckabee and Rick Santorum joined the billionaire frontrunner at the benefit event that was held at the same time as the top tier Fox News Republican debate, The Christian Post reported.

Texas senator Ted Cruz, locked in a tight contest with Mr Trump in Iowa, opened the Fox News debate with a sarcastic impression of the property mogul's frequent insults of his opponents.

But Trump has a genuine battle on his hands in Iowa with ultra-conservative Cruz who trails by about five percentage points in the RealClearPolitics average of recent Iowa polls.

"They didn't even mention he was born in Canada", Trump lamented. That debate had an audience of around 25 million viewers, so approximately double Thursday night's viewership.

Trump noted that skipping the debate was a "risk" given that other candidates could have shifted the dynamic of the race in Trump's absence.

"Ted Cruz may not be a USA citizen, right?" Paul did however participate in Thursday night's Fox News debate after a surge in Iowa polling caused him to qualify for its main stage.

"The debates are a awful waste of money, we don't need any more", said Sidney Williams, 75, who traveled to the event from Wellesley, Mass., donning a Trump pin.

Thursday's debate drew 12.5 million total viewers, according to Nielsen, down from the 24 million who watched the last FNC debate, which was also the first of this election cycle. "I'm glad I wasn't there", Trump said to an amused crowd. "And Ben, you're a awful surgeon", Cruz said, looking mischievously at retired neurosurgeon Ben Carson. Donald Trump (@realDonaldTrump) tweeted.

Despite the Trump event's lousy viewership, his ratings prophecy was at least partially correct: The Thursday night debate was the second-lowest rated this season. Trump, the statement said, "offered to appear at the debate upon the condition that Fox News contribute $5 million to his charities". Overall, ratings for prime-time Republican debates - fueled by Trump's popularity and unpredictability - have been three to four times higher than they were in 2011.