Jeb Bush, then governor of Florida, greets his brother George during a presidential trip to Miami in 2006. (Joe Raedle/Getty Images)

The 2016 campaign has bewildered and captivated George W. Bush. At home in Dallas, the 43rd president rises before dawn and reads political news online. He fires off emails to his old advisers to check on the latest campaign-trail ­gossip. He tunes into the debates, even though they stretch past his bedtime.

In private and among friends, Bush and his wife, Laura, express amazement at an election season that has been hijacked by Donald Trump. At a get-together last month, Clay Johnson, a lifelong friend, recalled that he and Bush said to each other, “Can you believe what’s going on?”

“He, like everybody else in America, is taken aback,” Johnson said. He and Bush chewed over the race for 30 minutes, including the rise of Bernie Sanders on the Democratic side. Johnson said Bush told him he thought expectations for his brother, Jeb, were so low that he could rebound by springing a surprise in an early state — a place like South Carolina.

On Monday — Presidents’ Day — Bush will try to help Jeb do just that. After deliberately avoiding politics throughout his post-

presidency, Bush is stepping back into the arena for an evening rally in North Charleston with his brother, the former Florida governor.

Bush’s appearance is an urgent mission to energize Jeb’s struggling candidacy ahead of next Saturday’s South Carolina primary. But it also will firmly put the stamp of the last Republican to occupy the White House on the GOP contest at a moment of deep division.

Former president George W. Bush appears in an ad released by the super PAC supporting his brother, Jeb, for president in the 2016 race. (Right to Rise)

In the seven years since Bush left the White House, the party has evolved so much it is almost unrecognizable from the one he led in the early 2000s. The GOP is being torn apart by a battle between the business establishment, movement conservatives and pitchfork populists.

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The former president has been troubled by a Republican electorate that — so far, at least — prioritizes Trump’s anger and projected strength over Jeb’s qualifications and experience, friends and former advisers said.

“He knows that all the qualities it takes to be a good president are not necessarily all the qualities it takes to be elected president,” said Donald L. Evans, a confidant and former secretary of commerce. “Entertainment and show­­time is one thing, but being the leader of the free world, being the leader of this country in these troubled, difficult times, is something totally different.”

Monday’s rally will be Bush’s first public appearance of this campaign, although he has headlined private fundraisers and vouches for Jeb in television and radio advertisements now blanketing South Carolina’s airwaves.

George W. Bush is popular among Republicans here, especially members of the military services. Jim Dyke, a top Jeb Bush adviser in South Carolina, said the event could be a “breakout moment.”

The former president’s approval rating among South Carolina Republicans stands at 84 percent, according to a private poll commissioned two months ago by Katon Dawson, a former state GOP chairman. That would put him in a tie with Gov. Nikki Haley and Sen. Tim Scott as the most popular Republicans in the state.

Republican presidential candidate Jeb Bush speaks during a campaign event in Columbia, South Carolina February 11, 2016. (Chris Keane/Reuters)

“The question is: Is that popularity transferable?” Dawson said. “I don’t think it’s transferable if he just says, ‘Jeb’s a good guy. And trust him.’ I think it is transferable if George W. Bush comes here, takes the gloves off and starts getting at it against Trump.”

[For Jeb Bush, the challenge is making it about ‘Jeb’ and not ‘Bush’]

The 43rd president, however, is unlikely to attack any candidate.

“Having been in the Oval Office, you can expect him to be a statesman and focus on extolling the virtues of his brother and sharing observations about what he thinks the requirements are for the next president,” said Karl Rove, Bush’s longtime chief strategist.

Anita McBride, a longtime Bush family aide, said George W. Bush “is dying to get out there,” adding: “He doesn’t want to hurt his brother; he wants to help his brother.”

For Jeb, there are risks to the joint appearance — a visceral reminder that electing him would continue a dynasty.

Some activists on the right think George W. Bush was insufficiently conservative as president, citing the 2008 bank bailouts and his advocacy for a pathway to citizenship for illegal immigrants.

Asked whether Bush still held grass-roots sway, former Sen. Jim DeMint (R-S.C.) said: “I have no idea. I really don’t. It could go either way, I guess.”

Pollster Kellyanne Conway, who leads a super PAC supporting Sen. Ted Cruz (R-Tex.), was skeptical. “This is a movement who’s always looking for the next Reagan — and let’s face it, if you’re looking for the next Reagan, why keep picking new Bushes?” she said.

If past is prologue, Trump will use Bush’s appearance as ammunition to torment Jeb. Last week in New Hampshire, Trump mocked Jeb for campaigning with his 90-year-old mother, Barbara Bush, the former first lady.

[Flashback: ‘I love my father and my brother...but I am my own man.’]

Rove argued such a swipe would backfire. “If any candidate decides they’re going to score points by going after 43, it’s going to be a problem for them — a mistake in judgment,” he said.

Among the general electorate, George W. Bush is a polarizing figure — especially because of the instability that followed the war in Iraq.

Still, Bush’s friends think that the passing of time has generated warmer feelings, as it has for his father, former president George H.W. Bush.

Jeb Bush has limited practical foreign policy experience, but he is cloaking himself in his family’s experiences. He tells crowds that if they liked how his father helped end the Cold War and how his brother kept them safe after the Sept. 11, 2001, terrorist attacks, then they’ll like him as commander in chief.

He said Friday on Brian Kilmeade’s radio show that he was “honored beyond belief” to campaign with his brother. “Donald Trump and anybody else who wants to be critical of George W. Bush, I hope they do it, because the people of South Carolina are sick and tired of people tearing down George W. Bush,” he said.

The entire Bush family has been actively engaged in Jeb’s campaign. Barbara Bush appeared alongside Jeb at events in New Hampshire. Previously unreported, however, is that the former first lady also worked the phones on her son’s behalf. As her friends knocked on doors in New Hampshire last weekend, if they found an undecided voter, they would call the family matriarch back in Houston and put her on the phone. In many cases, she sealed the deal.

“They were just sort of blown away by it,” said Jean Becker, the chief of staff to George H.W. Bush.

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The couple are keeping close tabs on their son’s campaign and plan to attend a Feb. 25 GOP debate in Houston. During ­lunches, George H.W. Bush often peppers Becker with questions.

“Who have you talked to? What do you hear?” she recalled him asking. “He wants all the inside scoop.”

As a politician, George W. Bush was a natural competitor and lived for the game. These days, he follows politics like a fan follows baseball. He doesn’t long to be back on the field, friends said, but stays abreast of the twists and turns.

There are moments that might have infuriated Bush, though he resists the temptation to interject. When Trump started taunting Jeb for being “low energy,” Bush held his tongue.

“Jeb doesn’t need big brother coming in to take on some other candidate,” said Evans, board chairman of the George W. Bush Foundation.

Josh Bolten, who served as Bush’s White House chief of staff, said: “It would be easy to get in the mix along the way and respond here and there, but that’s not who he is. He has too much respect for the office. . . . He said, ‘The right thing for me is not to be out there pontificating and trying to second-guess on a daily basis.’ ”

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Bush is also loath to critique Jeb’s campaign, friends said. The brothers, separated in age by six years, are not particularly close. They speak occasionally, but only when Jeb calls George. Sensitive to Jeb’s demanding schedule, George doesn’t want to pester. He offers advice only when Jeb asks.

Bush does not limit his counsel to his brother. When he was still running, New Jersey Gov. Chris Christie met privately with Bush to seek his advice about the presidency, according to people familiar with the meeting. But the list of candidates who have not paid Bush a courtesy visit is longer: Trump, Cruz, Sen. Marco Rubio of Florida and Ohio Gov. John Kasich.

When Bush gets together with friends, sometimes they go around the room making predictions about who the GOP nominee might be. Some say Trump; others say Rubio.

But one person always says Jeb.

“I see the path for Jeb to win. I believe Jeb’s going to win,” the former president tells his buddies, according to one of them, who requested anonymity to relate the private conversations.

“He believes it,” this friend said. “He totally believes it.”

O’Keefe reported from Myrtle Beach, S.C.