He’s the smart one.

That’s what Newt Gingrich has been hearing since he was little Newtie, the boy who, his stepfather said, had read much of the Encyclopedia Americana by age 12. He’s the smartest guy in the room, the professor-pol, the eccentric big thinker of the Republican Party with the near-textbook recall and the lawn-sprinkler spray of ideas.

Now, after a 13-year hiatus from politics, that pugnaciously cherubic face under the dome of white hair is back on the main stage. The 68-year-old former House speaker is climbing the polls as GOP voters watch him unspool seamless paragraphs on debate stages lined with stammering presidential opponents.

To some, a lifetime of hearing how smart he is has produced in Gingrich an ungovernable ego, colossal even by Washington standards. To others, it’s just biography. He vacuums information, and always has.

“One time we were outside and he was cooking hamburgers, talking to me and reading a book all at the same time,” said Joanne Harwell, 76, the wife of the pastor at First Baptist Church in Carollton, Ga., where Gingrich was a deacon and Sunday school teacher during his professor days at West Georgia College. “And it was a thick book.”

Evenings at the Gingrich home were often silent but for the sound of flipping pages, with his youngest daughter, Jackie, on the floor, sister Kathy draped across a chair, his first wife on the couch, all deep in a chapter of something.

“My dad would read, and he would walk slowly back and forth across the room,” said Jackie Gingrich Cushman, 45, from Atlanta. “He was a speed reader, and his finger moved along under the lines pretty fast. We spent a lot of evenings like that.”

It was a library hush that would not survive the barrage that Gingrich was about to launch on American politics.

If one hemisphere of the Gingrich brain is chock-full of facts, maybe it’s the other one that tosses the grenades. They are the double helixes of his persona, big thinking and bomb throwing, intertwined. Ever since he was brainy kid from an unsettled home, he has deployed intelligence and an embrace of disorder to vault himself ahead in life. He is a public-sector version of what business schools call a “disrupter.”

Gingrich is not so much a change agent as an upheaval agent. The man who shattered a 40-year Democratic House majority, upended congressional culture, transformed welfare, shut down the government, took on Big Bird — and along the way married three times and morphed from Lutheran to Southern Baptist to Roman Catholic — is no slave to the status quo.

To Gingrich, the thirst for roiled water is just a natural extension of the restless society he grew up in.

“Americans are astonishingly change-oriented,” he said last week in an interview at his austere campaign office in Arlington, talking between bites of cheeseburger from a foam box. “They go from BlackBerrys to iPhones. They go from iPhones to iPads. They go from movies to YouTube. It’s the most churning, chaotic society in history.”

And now he wants to be its churning, chaotic leader. His sudden success has legions of longtime Newtologists wondering which Gingrich will dominate his candidacy — or his presidency. The contemplative historian or the combustible politician? Or both?

Pete Wehner, a conservative commentator who has been watching Gingrich operate for more than 20 years, offers a mash-up theory of his rise.

“It’s all a combination of intelligence and ambition and drive and ego,” said Wehner, who worked for Ronald Reagan, George W. Bush and George H.W. Bush. “It’s wanting something and wanting it very badly and being willing to tear up the rules of conduct.”

For many Republicans, there’s a ­NASCAR quality to watching Gingrich reenter the fast lane of public life. It’s easy to admire the smart maneuvering while waiting for him to drive into a wall.

The education of Newt Gingrich began when his grandmother in Harrisburg, Pa., taught him to read well before school age. His aunts took him to zoos in Philadelphia and Washington, spawning not just a love of animals but also a taste for the bold stroke. Before he hit puberty, he walked alone into a meeting of city leaders to present his plan for a Harrisburg Zoo, complete with a fundraising plan that called for 300 donors to give $100 a month. They declined.

“I have an endless interest in everything,” he said. “My whole life has been one long fascination. And then I see something that needs to be improved or fixed or changed, and I try to see if I can help.”

Gingrich’s voracious hunger for reading material — biographies, history, science fiction — is legendary among his staff, as is his continuing interest in zoology, dinosaurs and space exploration.

“He devours Nature magazine,” said Rick Tyler, a longtime aide who was among those who quit the Newt 2012 team this summer out of frustration with the campaign’s early lack of discipline. Tyler has visited more than 35 zoos with Gingrich. He recalls a trip to New York’s American Museum of Natural History that turned into an esoteric rumination on evolutionary biology with researchers.

“At one point, they had just left the planet and I had no idea what they were talking about,” Tyler said, “but Newt was right with them.”

It’s easier to understand Gingrich’s hungry mind than his comfort with chaos. His friends say it’s important to consider just how often the disrupter’s own life was disrupted. His parents divorced almost immediately after his birth in Harrisburg in 1943. His mother soon married an Army colonel named Robert Gingrich, Newt McPherson became Newt Gingrich and the family hit the road. With each move — Kansas, France, Germany — the history buff became the new nerd at school.

“He was not born into any inner circle,” said Tony Blankley, a former Gingrich spokesman. “He was an Army brat from Pennsylvania who ended up at a high school in Columbus, Georgia, wearing glasses . . . in a town full of Southern jocks.”

But Gingrich never responded with meekness when audaciousness would better serve the ambitions of a young man in a permanent hurry. At 19, just weeks after graduating from high school, he asked Jackie Battley, his 26-year-old geometry teacher, to marry him. (She said yes.) One year after getting his first faculty job at West Georgia, he applied to be chairman of the entire history department. (They said no.)

At Emory University, he started a chapter of the College Republicans. At Tulane, working on his PhD, he campaigned for liberal Republican Nelson Rockefeller in 1968. Working the hippie-era campus in a coat and tie, he generated a crowd of hundreds to meet the candidate’s plane.

“He had an ability to get people involved in things,” recalled Kit Wisdom, who headed the campaign at Tulane. “It was obvious he was going to run for something himself.”

Gingrich began running for something almost as soon as he arrived at West Georgia College to teach history in 1970.

“He was sort of universally considered a character at West Georgia, smart but not very deep,” said New Republic columnist Ed Kilgore, a Georgia Republican-turned-Democrat who has known Gingrich for decades. “He clearly loved the professor’s persona and grand ideas, but he was more nakedly ambitious than anything. You always had this odd combination of Boss Tweed and Metternich.”

Gingrich lost two campaigns for Congress running as a moderate, environmentalist Republican. Then, in 1978, he ran for a third time as a full-throated movement conservative, attacking his Democratic opponent as a big-government liberal and railing against welfare.

He won and moved his family to Washington. Soon his marriage fell apart — the first of two divorces, each with accusations of infidelity.

Jackie Gingrich had campaigned tirelessly for her husband. The split-up came as a shock to their friends.

“When he came to see us that weekend, I stayed up with Newt all night and we talked,” Harwell, the pastor’s wife, said of a 1980 visit by Gingrich in which he told his friends of the divorce. “He told me he did not need a mother any longer, that she was holding him back.” She and her husband were offended. “We’d had a very close relationship with Newt, but after that the friendship ended.”

Gingrich remembers long talks with Harwell and said he would not have made such statements. He would not discuss reasons for the breakup, and a call to his ex-wife’s home was returned by a Gingrich campaign spokesman, who said she had no comment. But their daughter said it was her mother’s decision to end the increasingly troubled marriage.

“What she tells me was that she asked for the divorce because she thought that was what was best for her and her daughters at that time,” Jackie Cushman said. “Was it hard on my mother? Absolutely, but we all moved on.”

In recent years, her parents have attended the same family dinners for their grandchildren, Cushman said, and her mother often baby-sits when she is off campaigning for her father.

Gingrich was remarried six months after the divorce was final, to Marianne Ginther, an Ohio woman 15 years his junior. They divorced in 2000 after he began a relationship with Callista Bisek, a Hill staffer. He married the third Mrs. Gingrich shortly after his second divorce.

But it was always in Washington that Gingrich created the greatest convulsions. Weeks after his 1978 election to the House, he began trumpeting long-simmering accusations of corruption against Charles Diggs, a Detroit Democrat.

“Even before he was sworn in, he was making noises about going after Diggs,” said Craig Shirley, a Republican consultant and biographer whose book “Citizen Newt” will be out next spring. “From the very first, he stood out as someone willing to shake things up.”

There was no other way, Gingrich explains now, to shock a pulse into the comfortably comatose Republican caucus, long a cowed minority.

“You had a leadership that was exhausted and defeated by Watergate,” he said. “They weren’t prepared to fight.”

Thus began his greatest disruption, his years-long siege to take over the House. It was the ultimate combination of brains and brazenness.

He launched a sustained harangue on C-SPAN against the culture of corruption in the entrenched majority. And he filed ethics charges against three more Democrats, including Speaker James Wright over his use of book sales to skirt financial restrictions. Wright resigned in 1989. Eight years later, Gingrich himself would be reprimanded, and fined $300,000, for misleading an ethics committee investigation about his use of a college course for political fundraising.

In 1994, after Gingrich’s “Contract With America” had Republicans nationwide reading from a single set of conservative talking points, the party won its first House majority in 40 years. Gingrich was rewarded with the speakership. Immediately, with a deft blend of combat and compromise with President Bill Clinton, he began to pile up a remarkable record of conservative achievements, including welfare reform and a string of balanced budgets.

But sometimes when Gingrich hits the plunger, what blows up is Gingrich. Jack Howard, Speaker Gingrich’s director of policy, remembers a 1995 meeting in which Gingrich said he was prepared to shut down the government if Clinton didn’t give more ground in budget talks.

“We all thought it was pretty radical,” recalled Howard, a lobbyist at Wexler and Walker. “But we thought we had the high ground in terms of the issue.”

When Clinton didn’t blink, the shutdown turned into a PR disaster.

After just four years, disenchantment with Gingrich’s style brought an end to his remarkable run with the gavel. He survived a coup attempt by members of his leadership team but resigned from Congress after Republicans lost seats in the 1998 election.

His caucus was burned out by the nonstop crisis and the off-the-cuff bombast that detonated in the middle of a news cycle. Gingrich’s brain needed a 10-second broadcast delay.

“He couldn’t walk to his car without people peppering him with questions,” said Christina Martin, his press secretary at the time. “What he started to learn was you don’t have to say something every time you walk through Statuary Hall.”

Can Gingrich separate his two selves, the brainy and the ballistic? His daughters and former aides say his grandchildren, his marriage to Callista, 45, and his conversion to Catholicism have made him less prone to pull the pin.

 “I do not think the Newt Gingrich of the 1990s is the Newt Gingrich of 2012,” Martin said. “I think he is more mature. He’s a bit more tempered than he has been in the past.”

 But his run for the GOP nomination started with signs that there would be no new Newt. He infuriated conservatives by characterizing Rep. Paul Ryan’s proposed Medicare overhaul as “right-wing social engineering.” Since his political resurrection, he has challenged child-labor laws as “truly stupid” and dismissed Palestinians as an “invented” people.”

But still, in the chaotic political environment that is Gingrich’s natural habitat, he is on top, a surge built on his debate performances and the GOP electorate’s hunger for someone to shake things up.

Last month, he began a campaign swing with a boisterous tea party rally in Florida and ended with a murmuring seminar of graduate students at Harvard.

“If we plan correctly, by the time President Obama lands in Chicago [on Inauguration Day 2013], we’ll have dismantled about 40 percent of his government,” the GOP’s disrupter assured the cheering crowd outside a Jacksonville shopping mall. A man waved a sign reading, “Yes We Can Defeat Obama’s Socialist Transformation of America.”

One day later, in Cambridge, Mass., the only sign was the tasteful stenciling on the wall, “John F. Kennedy School of Government,” and Professor Gingrich was lecturing young academics on his theories of American exceptionalism.

“I disagree with everything he said,” said Harvard Law graduate student Jennifer Devlin. “But he handles himself brilliantly. He’s clearly very intelligent.”

Yes. He has heard that before.