But perhaps the biggest challenge for the former Florida governor is himself.
After a flat performance in the first debate and a slide backward in national and early state polls, the former Florida governor has taken to calling himself the "joyful tortoise" of the race, a role that many did not expect the candidate with more than $100 million to be playing. But at a time when Republican voters are looking for a candidate to channel their anger and frustration, Bush's bookish -- sometimes awkward -- demeanor is preventing him from connecting with wide swaths of the primary electorate.
He refuses to compete with Trump's antics and swatted away questions about his current position in the race during his recent visit here.
"This is a long haul man," he told a reporter as he dropped a corn kernel into his half-empty jar at the Iowa State fair's corn kernel poll. "Slow and steady progress."
Friends and aides describe Bush as thoughtful, tough, and exacting, qualities they say make him a great leader. But those traits don't always translate well to the demands of a campaign driven by social media hijinks or to the glad-handing that takes successful candidates across the finish line.
A day before Donald Trump breezed through his hour-long visit to the Iowa State Fair, waving quick hellos from inside his buffer of a dozen private security guards with the bearing of an imperial candidate, Bush clocked four hours of aggressive campaigning -- relishing the chance to engage with voters in policy discussions.
After meeting him, voters invariably used words like "warm," "direct," "genuine," and "earnest" to describe the former Florida governor. Few doubt his qualifications or readiness for the intellectual rigors of the Oval Office.
But they are rarely dazzled in the same way they are by rival candidates like Trump, or Florida Sen. Marco Rubio, or even by Bush's brother, George W. Bush, who could lock eyes and grip the shoulder of a voter for seconds on the rope line, and leave them with a sense of a deep, personal connection.
For all his tortoise analogies, there is a restless quality about the younger Bush -- a sense that he is always in a hurry to get to the next milepost, even at venues as laid back as the Iowa State Fair.
At 6'4 and feeling "a little taller," he said Friday, in his black cowboy boots with "Jeb" emblazoned on the side, he moved briskly across the fairgrounds when other candidates might have strolled.
Many fairgoers came up to express their admiration for Bush's father and his brother. At one point, Bush told a voter clutching his mother's book that he had sobbed when listening to one part of the narrative on tape. (His mother, he added, didn't believe him).
But he seemed most engaged when confronted by substantive questions. When a young woman asked for a selfie and his immigration proposal, he screeched to a halt to tell her about his "five point plan" -- ticking off each of the five elements on his fingers in depth.
While wielding a spatula at the pork producer's tent where tenderloin was sizzling on the grill, he brushed off a chance for light banter when someone asked whether he or his brother was a better grill master. ("I don't know how he is on the grill," he replied).
But standing next to Iowa Gov. Terry Branstad in his red apron, he dove into a wonkish policy discussion about an Iowa challenge to the federal regulations for agricultural runoff and the water quality rules imposed by federal officials.
"That should be a state issue wherever possible," Bush said, sampling his own pork chop on a stick while keeping a careful eye on the tenderloins he was flipping.
He effortlessly listed the average annual inches of rainfall in Florida, the attributes of Florida's "fragile ecosystem," and cited several examples of how runoff issues vary among states like Florida, Mississippi and Iowa.
"This administration has gotten really hyperactive in expanding dramatically what the definition of federal waters are. To get a permit to put a culvert in for your field? Or to get a permit for a drainage ditch or a drainage pond?" he said with indignation.
"They impose their will top down," he added, promising to reverse the rules. "The 10th Amendment is just as important as all the other bill of rights and we've trampled over it."
He is not in a hurry to give answers to questions that he is still thinking about. He paused for a long while after a reporter asked him Thursday night in Iowa whether he would like his own helicopter.
"There's no good answer to that," he replied after a beat, before moving on.
He also does not pretend to enjoy the more tiresome aspects of campaigning like the enormous press scrum that formed around him as he passed through the fair. When a photographer accidentally backed into a man in a wheelchair, Bush came to a dead stop in the street and turned to an aide: "Have they no shame?" he asked.
Flashes of impatience also flicker across his face if he doesn't like a question from a reporter, a trait his advisers hope to keep check in during the upcoming series of presidential debates.
At the same time, while he is criticized as rusty after more than a decade away from the campaign trail, he also demonstrates a style that is careful and self-aware as a campaigner, which can be an asset over the long run.
After ordering a fried Snickers at the fair, he ate it in a fashion that showed he had put thought into avoiding embarrassing photos -- taking a bite from the middle, not the end, and quickly dusting the powdered sugar off his mouth with a napkin.
Making the sale
Voters seem to like Bush after meeting him, but time and again after his events here Thursday and Friday, he had moved on before making the sale.
"He seems smart and capable," said 69-year-old Jerry Litzel, who came to hear Bush at the Polk County Summer Sizzle Thursday night. "But he's soft-spoken. I think we need someone who can stand up for the United States." (Trump was among the candidates Litzel said he was considering).
Robert Davis, a hairdresser from Des Moines who buttonholed Bush in the state fair's "Bud Tent" to tell him he was going to be the next president, was blunt when the candidate walked away.
"He needs to get his ball rolling," Davis said of Bush. The hairdresser is considering the former Florida governor along with Trump, who he likes because "he says what we think, out loud."
"You have to bring people in. You have to make them respond to you -- that is what Trump is doing," Davis said. "Bush hasn't showed that yet."
That is a worry even for voters who say Bush is their top choice, like Iowan Barb Wine, a 64-year-old Amway Distributor who also met the former Florida governor in the beer tent Friday.
Moments after Bush had sipped "a cold one" at 10:40 a.m., Wine asked him point blank to explain how he was different from Trump.
He replied that he was a consistent conservative with a proven record.
"I don't assume it's my way or the highway," Bush told her. "I've got deeply held views, but there's a big difference -- I think you can pretty well see it, just in demeanor. I want to broaden out our message to win a larger number of people."
"I respect him and clearly he's made great progress in his campaign, but clearly we're very different. Ultimately, people are going to look at it and notice."
But "he's called you kind of a milquetoast?" Wine pressed Bush, "How do you respond to that?"
"Yeah, well, I go campaign," Bush replied. "I'm going to do it hard, campaign all over -- and I'll turn people towards us.... At some point, you've got to be substantive about what you're going to do."
Charm on the trail
For all his seriousness, Bush is not without his moments of charm on the campaign trail.
As he headed out of the fair Friday, he caught a bag of caramel corn that Glenda Hockridge, tossed to him from inside her cotton candy truck.
He circled back and insisted on paying. She refused, stating it was from the "goodness of her heart." But he wouldn't take 'No' for an answer, slapping the dollar bills on the counter and kissing her hand before darting away.
Hockridge was still beaming an hour later. The North Dakota independent said she was deciding between Bush and Hillary Clinton, and described Bush as "down to earth" and "understanding" of hardworking people like her.
"If he goes around kissing people's hands, his wife is a very lucky woman," Hockridge said. Asked her age, she demurred: "As long as a man kisses my hand," she said, "I'm 29 and holding."