Donald Trump keeps calling out his opponents for one personality trait

Donald Trump doesn't think many of his Republican rivals have the energy to be president.

In a brief speech in Iowa on Saturday, the current Republican front-runner told the crowd that he doesn't think retired neurosurgeon Dr. Ben Carson would be an effective negotiator because he lacks the energy to make a good deal.

"I don't think Ben has the energy," Trump said. "Ben is a nice man, but when you're negotiating against China and you're negotiating against these Japanese guys that are going to come against you in waves, and they think we're all a bunch of jerks because our leaders are so stupid and so incompetent and so inept, we need people that are really smart, that have tremendous deal-making skills and that have great, great energy."

In an interview on ABC's This Week on Sunday, Carson defended himself, saying that during long surgeries, he was able to make "critical decisions after many hours of intense work."

The retired neurosurgeon also took a shot at Trump.

''You don't have to be loud to be energetic," Carson said.

Carson isn't the first presidential contender that Trump has criticized for lacking energy - Trump has employed the insult time and again on several other presidential contenders, especially former Florida Gov. Jeb Bush (R).

Trump has repeatedly emphasized that the governor is a "low-energy person."

After Bush released a web video criticizing Trump's past ties to the Democratic party, Trump said that the campaign was forced into the position because Bush lacked the energy to attract primary voters.

"Well, I think he had really no choice. He's doing very poorly in the polls, he's a very low-energy kind of guy and he had to do something, so they're spending a lot of money on ads," Trump said in an interview on Good Morning America.

The attack appears to have gotten under Bush's skin.

At a campaign event earlier this month, the former governor - who once described himself as a "joyful tortoise" in the long slog of a presidential campaign - touted his busy schedule as a clear sign that he is an energetic person.

"I'll just give you a little taste of the 'low energy' candidate's life this week," Bush said, before listing off the locations that he'd be traveling.

Shortly after declaring his candidacy, Trump said former Texas Gov. Rick Perry (R), who recently dropped out of the presidential race, didn't have the necessary "energy" and "brain power" necessary for the presidency.

"I think that he's trying so hard," Trump told the Daily Mail. "But it's not about trying. It's about energy, it's about brainpower, it's about toughness."