The latest knock? Trump's oft-used "low energy" insult.

Speaking at a campaign rally in Boone, Iowa, Trump discussed one of his classic stump-speech topics -- his front-runner status in the polls. He dismissed the idea that Carson was gaining on him.

"Ben Carson is a very nice man. He's surging, but he's way behind me," Trump said. "The headline is, 'Carson surging.' I keep saying, 'What about Trump?' ... I'm surging much more than him, I'm surging more than anybody."

As he continued his riff on Carson, Trump criticized the neurosurgeon for not having what it takes to be president.

"We need energy," Trump said. "I don't think Ben has the energy. Ben is a nice man, but when you're negotiating against China, and you're negotiating against these Japanese guys who are going to come at you in waves ... we need people that are really smart that have tremendous deal-making skills and have great, great energy."

Trump has also knocked former Florida Gov. Jeb Bush -- another top competitor to Trump for the nomination -- as having "low energy."

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For weeks, Trump and Carson were very complimentary toward one another, until Carson seemingly implied this week that Trump's Christian faith was disingenuous.

"Ben hit me on my faith, and you don't hit a person on faith," Trump said Saturday. "I don't know him; he knows nothing about me. I'm Protestant; I'm Presbyterian. I believe strongly, I believe in the Bible strongly."

Carson later apologized, saying his remarks were misinterpreted.

"He was nice enough to apologize," Trump said, "so I respect him." But he later continued to attack Carson.

Carson and Trump have both picked up momentum in polls in recent weeks, with Carson posting bigger gains than Trump. Carson tied Trump in Iowa in a Monmouth poll at the end of last month, and a Quinnipiac poll of Iowa this week showed Carson in second place, 6 points behind Trump, within the margin of error.

The two are competing for the evangelical vote in the key caucus state of Iowa.

Trump told the media that all the candidates who have attacked him have suffered in the polls, including former Texas Gov. Rick Perry, who dropped out of the race on Friday.

"He's a nice man, I knew him before he went a little bit hostile, and he went hostile to try and get a little bit of traction," Trump said. "It didn't work. Actually, so far, it hasn't worked for anybody."