"I could stand in the middle of 5 Avenue and shoot somebody and I wouldn't lose voters", Trump said to laughs and cheers from the crowd.

Trump has been leading national polls of Republican voters for months and now holds a lead in the early voting in Iowa caucuses that will be held on February 1.

He said American Bridge is "adjusting its program accordingly", meaning it is targeting Trump more forcefully.

GOP presidential front-runner Donald Trump reacted Saturday night on Fox News Channel's "Justice" to talk radio host Glenn Beck endorsing Sen.

"If Donald Trump wins, it's going to be a snowball to hell", Beck said. Trump has responded by criticizing Cruz for failing to work with his colleagues in the Senate.

Mr Cruz blasted Mr Trump's past reluctance to strip money from Planned Parenthood and cast the billionaire's plan to deport more than 11 million people who are in country illegally as "amnesty", because he would then let many of them return.

"I went into the hotel that I stayed in here last night, which was a very nice hotel, and I'm watching all these ads", Trump said. But in Iowa, it was immigration that birthed the most explicit, concentrated contrast Cruz has offered on the stump since he shredded the non-aggression pact that the pair had essentially signed during Trump's rise. "I like people that weren't captured, OK?"

"I've spent the least amount on the campaign and I'm in first place".

Some party stalwarts however have welcomed Trump. Veteran senator Bob Dole, the party's presidential candidate in 1996, has the same opinion. "It's been fun. But Iowa, I bet you, I beg you, my children's future depends on what you do a week from Monday", he said. "It's, like, incredible", he said just before making the homicide remark.

"He pointed out some things maybe in Donald Trump's history", Claassen said, "that don't sit as well".

Meanwhile, the Democratic frontrunner Hillary Clinton - as well as Republican candidate Marco Rubio - received endorsements from Iowa's largest newspaper.

A Cruz-Trump battle that turned ugly after the two dissolved their short-term "bromance" - a period marked by relatively nonhostile politeness - could get even uglier as the campaign turns to Texas, if both candidates are still in play.