Donald Trump is constantly repeating this Jeb Bush zinger

Real-estate tycoon Donald Trump has settled on one go-to insult when it comes to his closest rival in the Republican presidential primary.

Almost every recent time Trump addressed former Florida Gov. Jeb Bush (R) on the campaign trail, the businessman attacked Bush as a "low-energy person."

"Jeb Bush is a nice person. He doesn't have the energy or the capacity to make our country great again. That I can tell you 100%," Trump said Monday night on Fox News. 

During a Monday interview with The New York Times, Trump repeated the phrase "low-energy person" several times while describing Bush.

Before that, at a campaign rally in Alabama on Friday, Trump labeled Bush a "very, very low-energy person."

"Let's assume that somebody else becomes president. Wouldn't that be horrible? Wouldn't that be horrible? So let's assume that somebody else becomes president. Let's assume a very low-energy - very, very low energy, so low energy that every time you watch him, you fall asleep - [person]," he said. "Let's say Jeb becomes president. Not good. Not good."

At a Wednesday rally in New Hampshire, Trump again called Bush a "low-energy person," according to NPR. During a Time magazine interview that week, Trump called Bush a "very low-energy person" when asked about one of the former governor's foreign policy critiques.

And so on. 

Even Roger Stone, a former Trump adviser who frequently appears on television following his departure from the campaign, echoed the anti-Bush talking point. 

"That Paleo diet - get this guy a cheeseburger," Stone declared during a CNN interview last weekend, referencing Bush's Paleolithic diet that excludes processed foods. "He's got no energy. He's just flat. There's no passion there."

Business Insider reached out to Bush campaign spokesman Tim Miller about the "low energy" criticism. Miller pointed to recent coverage of Bush's hands-on response to devastating hurricanes that hit Florida while he served as the state's governor.

It's not clear what prompted Trump's attack against Bush's alleged lack of energy, but the two men have been increasingly exchanging shots at one another on the campaign trail in recent days. Previously, Trump mostly went after Bush's positions on immigration and the Common Core educational standards, two standard intra-party lines of attack against Bush.

Last Wednesday, Bush started openly questioning Trump's conservative credentials.

 "Mr. Trump doesn't have a proven conservative record. He was a Democrat longer in the last decade than he was a Republican. He has given more money to Democrats than he's given to Republicans," Bush said. "When people look at his record, it's not a conservative record." 

 The Bush campaign went on to promote Bush's criticism to reporters and on social media:

The very next day, Bush's campaign fired back at Trump's Alabama rally.

"You may have heard Donald Trump is making a stop in Alabama tonight. Beneath Trump's bluster is a record that would make Hillary Clinton proud," the Bush team wrote to his Alabama supporters, according to an email that was forwarded to reporters.

The emailed continued: "Trump's positions are deeply out-of-step with the Alabama way of life. We know Alabama cherishes life, especially the life of the unborn. Not a single Alabama taxpayer wants to see a massive tax increase. Trump's history of supporting Democratic ideas will not go unnoticed in Alabama so please share with your friends."

On Monday, the Bush camp continued their counter assault against Trump and released a video slamming the developer's "catastrophic" immigration plan amid Bush's trip to the border town of McAllen, Texas.

Trump's response: "The last thing this country needs is a low-energy president."