Republican presidential front-runner Donald Trump has a new target, calling fellow White House contender Ted Cruz "a little bit of a maniac".
The jibe came as the US senator passed Trump in an Iowa opinion poll.
Cruz's dogged pursuit of conservative Iowa voters has paid off in the form of a 10-percentage-point lead over Trump in the state, where the first presidential nominating contest will take place on February 1.
Unlike the other Republicans seeking their party's nomination for the November 2016 presidential election, Texan Cruz has embraced Trump and avoided publicly criticising the popular candidate.
But last week, he questioned Trump's judgment at a private fundraiser, according to the New York Times, after the billionaire developer 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 on Fox News Sunday.
"I don't think he has the right temperament. I don't think he's got the right judgment. When you look at the way he has dealt with the Senate, where he goes in there like a, you know, frankly, like a little bit of a maniac - you are never going to get things done that way."
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 had a lighthearted response to the "maniac" label on Twitter later on Sunday, posting a link to a video clip from the 1983 film Flashdance showing star Jennifer Beals dancing energetically as the hit song Maniac plays on the soundtrack.
"In honor of my friend @realDonaldTrump and good-hearted #Maniacs everywhere," Cruz said in his tweet.
Cruz rose to 31 per cent, above Trump's 21 per cent, in an Iowa poll released on Saturday by the Des Moines Register and Bloomberg News. That is a 21-point jump from October.
His rise came at the expense of retired neurosurgeon Ben Carson, who dropped to third with 13 per cent in the poll.
