But at CNN's Republican debate Tuesday, the former Florida governor tore into the GOP front-runner, solidifying himself as the establishment's primary attack dog against Trump.
While Bush has been willing to bash Trump for months -- firing off one of his biggest shots last week when he called his rival "unhinged" for proposing a ban on Muslims entering the country -- Bush's Tuesday debate performance showcased a new turbocharged offensive against the billionaire businessman.
So why does his campaign feel the new strategy will now work?
As one adviser put it, times have changed after the terror attacks in Paris and California, and Tuesday night was the first time that Bush could draw a serious contrast with Trump's "erratic" statements in front of millions of voters.
"We're looking at different stakes in terms of the commander in chief test," said Bush adviser Michael Steel.
Taking on the 'chaos candidate'
With more bravado than he's shown in previous debates, Bush won acclaim for his mostly smooth, confident delivery. A self-described policy wonk who's known to be long-winded, Bush spoke in sound bites and injected himself into conversations to get more air time.
But a large amount of that time was spent trying to frame Trump -- whom he called "Donald" all night -- as a flippant contender unfit for the oval office.
"He's a chaos candidate," Bush said. "And he'd be a chaos president. He would not be the commander in chief we need to keep our country safe."
He called Trump's proposal to kill the family members of terrorists "crazy" and dinged him for saying he gets his policy advice from watching "the shows."
"I don't know if that's Saturday morning or Sunday morning," Bush said to laughs, referring to Saturday morning cartoon shows.
At one point, the two tangled after Bush tried to interrupt Trump.
"Am I talking or are you talking, Jeb?" Trump said with irritation.
"I'm talking right now," Bush replied. "I'm talking."
After some more sparring, moderator Wolf Blitzer allowed Trump to continue, as a snarky Bush uttered, "A little of your own medicine there, Donald."
"I know you're trying to build up your energy, Jeb, but it's not working very well," he fired off.
Later, when Trump tried to paint Bush as too dovish on the Syrian refugee issue, Bush hit back. "Donald, you're not going to be able to insult your way to the presidency."
Back in Miami, Bush's campaign blasted out email after email showing video of the former Florida governor tearing into his opponent, and his top advisers trumpeted Bush's blows in the spin room after the debate.
Will strategy pay off?
While some of Bush's donors have been pressing for an aggressive take-down of Trump -- who also won praise for his debate performance -- it's unclear whether that strategy will pay dividends for Bush.
Republican strategist Ford O'Connell said Bush's pushback was strong, but appeared to be a last-resort tactic.
"He sees the writing on the wall, so now he's throwing caution to the wind even with respect to Trump mania and is trying to pull every rabbit out of the hat," he said. "The question is will it resonate with GOP primary voters? One thing is for sure: if this were a general election, Jeb would be in far greater shape."
Malachi Boyuls, a Bush donor from Texas who sits on the campaign's national finance team, argued that Bush's attacks were more substantive than strategic.
"I'd doubt his strategy was as much to go in attacking Trump as it was to impress upon viewers and the other candidates that these are serious times and serious issues, and we need a serious candidate to lead our country," he said.
Tearing into Trump so forcefully can be a risky move and has yet to prove to be an effective way to gain momentum for a candidate. Former Texas Gov. Rick Perry staged an all-out front against Trump this summer, calling him a "cancer on conservatism," only to drop out himself just weeks later. Louisiana Gov. Bobby Jindal also sparred with Trump over the summer, calling him a "narcissist" and "egomaniac"
in an unsuccessful attempt to save his campaign.
Meanwhile, Ohio Gov. John Kasich and South Carolina Sen. Lindsey Graham also have seen their attacks against Trump fall flat. Others, like Sen. Ted Cruz of Texas, have been far more restrained, hoping to one day win over Trump's supporters.
But Fergus Cullen, former chairman of the New Hampshire Republican GOP and author of a new book on the state's first-in-the-nation primary, said it's "good politics" for Bush to target Trump.
"He's not competing for Trump's 30%," Cullen said. "He's competing for the 40% of the party that finds Trump repulsive and wants someone to punch him in the nose."
Trump was dismissive of Bush's blows during and after the debate, saying his opponent was only trying to pull up his numbers and chalked it up as simply a political tactic.
"Jeb has to do what he has to do. He's having a hard time in the polls. I'm not," Trump told reporters in the spin room after the debate. "I would have done the exact same thing."
While other candidates, like Kentucky Sen. Rand Paul, briefly took aim at Trump, Bush largely stood solo in his fight against the front-runner Tuesday night.
And the former governor seemed pleased with his performance, appearing in a round of early morning TV news interviews Wednesday to tout his offensive.
"I don't know why others don't feel compelled to point that out but I did," he said on CNN's "New Day.
" "I think I got a chance to express my views and compare them to someone who talks a big game but really hasn't thought it through."