Aides and supporters describe Bush as a tough and determined campaigner, whose civil demeanor shouldn't be mistaken for a lack of grit. To the extent that his tone has changed, advisers say, that reflects a growing urgency that hangs over all the candidates as voting draws nearer.
But Bush, who has presented himself to voters as the man most qualified for the presidency, is not a natural attack dog, and he has repeatedly seemed uneasy going after his rivals in a negative tone. The knock on Bush: He might be too nice.
Buddy MacKay, the Democrat who lost a 1998 governor's race to Bush, described the presidential candidate as an "honorable" man who is simply not "comfortable with going negative."
"I find myself, strangely, a little sympathetic with him," MacKay, who served two terms as Florida's lieutenant governor, said in an interview. "I think it's kind of a compliment that he's not comfortable trying to outdo some of the other people in the wild comments."
When Bush has flashed a more confrontational side this year, it has not gone smoothly. At a CNN debate in September, Bush demanded that Trump apologize to his wife, Columba, for making inflammatory comments about her Mexican heritage.
Trump refused. "I won't do that because I said nothing wrong," he responded. Bush dropped the issue.
At last month's CNBC Republican debate in Boulder, Bush delivered a stinging criticism of Rubio's poor attendance record in the Senate, comparing it to a "French work week."
Rubio swung back, accusing Bush of going negative because "someone has convinced you that attacking me is going to help you." Bush looked on at his protégé from behind a podium, seemingly unprepared to deliver another punch.
The exchange frustrated Bush supporters, who felt their candidate had been pushed into a more hostile tone than is natural for him. He has since hired media consultant Jon Kraushar to help him with debate preparations.
Some of his surrogates have openly described the exchange with Rubio as a clumsy moment.
"That was a mistake. I don't know who thought that that was a good idea," said former New Hampshire Sen. Judd Gregg, who recently announced his endorsement of Bush. "I don't think that was his character and you could see he was uncomfortable doing it."
On Monday, Bush's attempt to give a fiery response to the question of whether he'd kill baby Adolf Hitler
-- "Hell yeah, I would!" -- scanned as awkward.
'He was always a gentleman to me'
To those who have watched Bush for years, it's a familiar dilemma. Even his former adversaries from Florida politics describe Bush as a reserved and decent man, who was never at his best on the attack.
When he first ran for governor in 1994, challenging incumbent Lawton Chiles, Bush's harshest attacks on the Democrat backfired. In a high-stakes debate, Bush criticized Chiles as weak on crime and the death penalty.
Chiles, a relaxed and folksy politician, rebuked Bush for the tenor of his campaign, comparing his attacks to the famously negative "Willie Horton ad" run by a group supporting Bush's father's 1988 presidential run. Bush was clearly flustered.
Four years later, Bush ran again, defeating MacKay -- Chiles' 1994 running mate -- by a wide margin.
Rick Dantzler, a Democrat who ran for lieutenant governor in 1998 as MacKay's running mate, said he viewed Bush as someone who enjoyed governing more than campaigning. Going on offense against opponents, Dantzler said, did not seem to be part of Bush's DNA.
"I think that's unnatural to him and not what he wants to do or is comfortable doing," Dantzler said. "He was always a gentleman to me."
Still, Bush has shown frustration that his own qualifications seem to count for less this year. Running for president is "a different kind of hard than I expected," Bush told CNN's Jamie Gangel last month, citing candidates like Trump, who "have no proven record in public life."
Trump, meanwhile, has taunted Bush with claims the Republican is "low-energy."
Justin Sayfie, a former spokesman for Bush in his days as governor, described him as a "very, very competitive person" whose intensity would grow more visible.
"I would not want to be someone who's on the receiving end of Jeb Bush's competitiveness. It's ferocious when it needs to be," Sayfie said.
Keeping the focus on issues, experience
If Bush can appear to be out of his element under the debate lights, he seems to find his groove in town halls. At local diners and meeting halls, Bush appears to delight in fielding questions from voters, some of whom say they are undecided about who to support in the primary.
No topic is off limits and Bush routinely answers narrow policy questions in great detail.
These wonky interactions are a stark contrast to the campaign styles of Trump and another political novice-turned-frontrunner, Ben Carson.
Neither of the insurgent candidates has dealt with policy in any depth, as both have risen in the polls. Trump, in particular, has captivated voters with precisely the kind of angry and colorful remarks that Bush avoids.
"He's not deviated from or shifted his beliefs to try and curry favor with one element of the electorate over another," a top Bush adviser said. "He is who he is and that will be rewarded when voters go to the voting booth."
To Bush's admirers -- including some Democrats -- the candidate's best bet may be hoping that voters may give more weight to Bush's more conventional credentials when it comes time to vote.
Former Florida Sen. Bob Graham, a Democrat, said Bush's strongest asset was that he is "the kind of person who can go three or four levels -- or more -- deep on a subject."
"One hundred days from now, when I suspect the field will be significantly smaller than it is today, it'll be more to Jeb's expectations," Graham said.
"It'll play more to his strengths."