Cruz's dogged pursuit of conservative Iowa voters has paid off in the form of a 10-point lead over Trump in the state, which has one of the earliest presidential contests.

Unlike the other Republicans in the 2016 White House race, the US senator from Texas has embraced Trump and avoided  public criticism of the popular candidate.

But last week he questioned Trump's judgment at a private fundraiser, according to the New York Times, after the billionaire businessman advocated temporarily banning Muslims from entering the United States.

That got Trump's attention."I don't think he is qualified to be president," Trump said.

Trump touted his ability to get along with liberals and conservatives and said that was the hallmark of the "world-class businessman" he is.

Cruz rose to 31 percent, above Trump's 21 percent, in an Iowa poll released on Saturday by the Des Moines Register and Bloomberg News. That's a 21-point jump from October.

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