WATCH: Bush Comes Out Swinging Against Trump In Dueling NH Town Halls

DERRY, NH — (CNN) In dueling town halls about 15 miles apart, Republican presidential rivals Donald Trump and Jeb Bush traded insults in an evening of sparring, with Bush — for the first time in his campaign — showing a willingness to give it back to the current front-runner.

“I don’t see how he’s electable,” Trump said of Bush to reporters, shortly before taking the stage at his first-ever town hall event as a declared candidate in Derry, New Hampshire. “Jeb Bush is low energy person. For him to get things done is hard. He’s very low energy.”

Later, taking questions from the audience, Trump said, “You know what is happening to Jeb’s crowd right down the street? They’re sleeping.”

“He was supposed to do well in New Hampshire? He’s going down like a rock,” Trump said to laughter.

Bush, once the national front-runner, displayed a marked change in tone when it came to Trump during his own town hall in Merrimack, New Hampshire.

“Mr. Trump doesn’t have a proven conservative record,” Bush said. “He was a Democrat longer in the last decade than he was a Republican. He’s given more money to Democrats than he’s given to Republicans.”

Bush pointed out that not “that long ago” Trump was advocating for a tax on people who have more than $10 million in assets.

“That’s not a conservative value in my mind,” Bush said. “He was for a single-payer system for health care until very recently. Not a very conservative point of view.”

“Look, Mr. Trump has clearly got talent, there’s no denying that,” the former Florida governor said. “But when people look at his record, it is not a conservative record — even on immigration, where it’s, you know, look, the language is pretty vitriolic for sure. But hundreds of billions of dollars of costs to implement his plan is not a conservative plan.”

In the past, Bush has limited his attacks on Trump to more strictly policy disagreements rather than reaching back and parsing through his Democratic resume lines.

Trump, meanwhile, basked in his front-runner status.

“The only thing constant is Trump,” he told reporters, comparing his position to that of the other candidates who find themselves constantly swapping ranks down below.