Brad Parscale was sounding upbeat. He has a new company and, he believes, a brighter future.

Mr. Parscale, President Trump’s former campaign manager, said he was trying to move on from that bleak Sunday in late September when he made the national newscasts, after police were called to his home in Fort Lauderdale, Fla. His wife told officers he was inside the house, ranting, acting erratically, with a loaded and cocked gun.

Now he is turning to real estate and plans to buy houses and flip them, he said in an interview this month, something he said he was good at. He is also restarting his political consulting firm, Parscale Strategy, and trying to kick off a start-up called Nucleus, to process and analyze data for conservative politicians.

“I spent five years developing the only automated web-based ecosystem that connected all our departments and made our campaign the most efficient in history,” Mr. Parscale said. “And now I want to bring this technology to campaigns all around the world who are right of center.”

Once a midlevel marketing executive in San Antonio, Mr. Parscale rose to the president’s inner circle and was hailed, somewhat hyperbolically, as the tech genius whose social media savvy won Mr. Trump the 2016 election. Mr. Parscale became expert in making the Trump campaign messages — sometimes gut-churning and cruel, other times patriotic and nostalgic — go wildly viral, and his dark humor seemed in tune with Mr. Trump and his meme-making fan base.

But people who know and worked with Mr. Parscale say he grew too enamored with his proximity to power, and naïvely comfortable with his insider status, which rested on the whims of a mercurial president. When he was replaced as campaign manager in July amid questions about his stewardship, particularly his spending decisions, it was an embarrassing blow.

In recent phone interviews, Mr. Parscale, 44, said he felt demonized by the left, which accused him of digital dark arts he did not employ, and scapegoated by the right for Mr. Trump’s failed campaign.

“They can’t choose: Am I rich or am I poor? Am I dumb or am I smart?” Mr. Parscale said of his political adversaries.

He has toggled between frustration that he remains a source of public interest and an inability to stay away from the spotlight. After his personal issues burst into public, he retreated, telling people that he was happy to leave the rat race behind, and that at least he has options because he has money.

He said he had not gone into rehab, as had been rumored, and was not getting divorced. But he was angry about how things went down, and wanted to live “off the grid,” away from the glare of high-stakes politics.

“I’m done with that industry,” he said last month. “It’s a nasty industry. I’ve always been into homes. That’s where I’ve invested. And I have good taste.”

But his initial impulse to jettison politics altogether soon gave way to the gravitational pull of the game: In a conversation a few weeks later, he had changed his mind. He was starting Nucleus.

A Serendipitous Pairing

Mr. Parscale’s exit from the Trump campaign could hardly have been more horrifying. A police video from the afternoon of Sept. 27 showed Mr. Parscale — shirtless, barefoot, wearing a baseball hat and holding a beer — as he talked to the police after emerging from his home. A split second later, a police officer tackled him, smashing his shoulder and chest into Mr. Parscale’s hips, driving him to the ground with a thud.

A few minutes earlier his wife, Candice Parscale, in a swimsuit and a towel, had shown officers bruises on her arms, the body camera footage shows. She said her husband had caused the bruises, according to the police report. The video made the evening news shows and soon went viral. Mr. Parscale was taken to the hospital and released. His wife later recanted her statements from that day.

Had the tables been turned that day and it was not Mr. Parscale who was the subject of the story, perhaps if it were a Democratic operative who had been tackled instead, the video is just the sort of content that Mr. Parscale might have quickly pumped into the news ecosystem, the way he did on countless occasions for Donald J. Trump.

The story of how Mr. Parscale came to work for Mr. Trump is serendipity, plus a little of Mr. Parscale’s opportunistic savvy. He was already a successful marketing executive, well known in the business circles of San Antonio, when about ten years ago one of his clients was on a flight next to someone who was about to take a job working for the Trump family. The client jotted contact info on an airplane napkin, and soon Mr. Parscale was looped in to bid on some digital work for the family. He cut his rate to make sure he would get the job.

Mr. Parscale and the Trump family clicked, and when the presidential campaign started, he was the obvious choice to handle the website and digital advertising.

Another bit of good fortune for Mr. Parscale: He would inherit a data operation from the Republican Party that had been totally overhauled, and he had the perfect candidate to try out the new system. Mr. Trump had limited resources and few data ideas of his own. He did not have a big existing digital team. He just had Mr. Parscale, who had no experience in politics.

What Mr. Parscale had was the trust of the president’s family, and a keen sense of the president’s voice and fondness for discord, which he wasn’t afraid to exploit.

His lack of expertise made him especially open to a powerful tool for reaching voters: Facebook. While others spent on television ads and hiring huge teams, Mr. Parscale saw that Facebook ads were cheaper and radically effective at reaching Trump voters. He decided to lean on Facebook for analytics rather than hiring a large team of his own.

“What Brad did was say, ‘We’re not going to ever be able to build it, so we’re just going to outsource all this stuff to Facebook itself, and they’ll run our ad campaign,’” said Daniel Kreiss, an associate professor at the University of North Carolina and the author of “Prototype Politics: Technology-Intensive Campaigning and the Data of Democracy.” “That was Brad’s true innovation.”

His genius was in making provocative content, editing it into fast-moving clips and testing it quickly to figure out the right tempo and tone. He knew how to select the right music for the video, the right text for the meme (maybe different text in Florida than in Ohio), and then sending it full force into the nation’s bloodstream through Facebook.

James Barnes, whom Facebook sent to San Antonio to work with Mr. Parscale, said the campaign tapped into what worked very well on Facebook: messages that stir outrage, fear, panic and a sense of victimhood. That was the message of Mr. Trump’s campaign as well.

“A lot of Americans just found Trump appealing and the campaign had relatively good tools to figure out who responded to what,” said Mr. Barnes, who by 2020 had left Facebook and was working for a progressive nonprofit to defeat Mr. Trump. “That was it.”

Mr. Parscale pushes back on the idea that Facebook essentially ran the campaign, phrasing it more as a special partnership.

“We asked Facebook for a manual, and they provided us a human one, which was extremely helpful,” Mr. Parscale said.

He said his particular skill was in harnessing the emotional charge of the Trump campaign, translating the rage and nostalgia into content that would spread.

“Americana worked,” he said. “Just Americana. ‘Bring back that America pride’ worked. Pictures of a space shuttle. Half my ads just look like a Fourth of July party with a Vietnam vet. I wasn’t some mad genius.”

A ‘Fetishization’ of Data

In the shock of Mr. Trump’s 2016 win, liberals and pundits wanted to know how it had happened and looked toward Silicon Valley. Somehow, they said, Americans must have been tricked into that vote. A mystique grew around Mr. Parscale.

“Secret Weapon,” announced CBS News. “Brad Parscale, digital director for Trump’s campaign, was a critical factor in the president’s election. Now questions surround how he did it.”

“There’s a fetishization of data that allows normally smart people to stop thinking and accept the words of a digital shaman,” said Ben Coffey Clark, a founding partner at Bully Pulpit Interactive, which advises Democratic campaigns. “Why was Brad so confident? Because he didn’t know any better.”

Regardless of how much digital genius was really there, Mr. Parscale’s power grew after 2016.

He knew how to navigate the turbulent currents of the Trump family. As Mr. Trump looked ahead to the 2020 election, he chose Mr. Parscale as the 2020 campaign manager.

By this time, former colleagues say, Mr. Parscale had developed an inflated sense of his importance. He would tell people that he and Hope Hicks, the president’s close adviser, were part of a small group of nonfamily members on a text chain with the Trump children. Mr. Parscale prided himself on being one of the few people who could tell the president bad news, and that he couldn’t be cut out because of his loyalty.

He saw himself as a campaign manager but also something more: a partner to Jared Kushner, the president’s son-in-law, who was overseeing the campaign from the White House, and he enjoyed the limelight enough that he would take the stage at Trump rallies and throw red MAGA caps into the crowd.

His Instagram feed was filled with pictures not of the candidate whose campaign he was running, but of himself, posing for selfies with fans or signing caps with a black Sharpie like the boss.

But in the summer, as the campaign stumbled, Mr. Parscale fell out of favor. In a particularly embarrassing moment, teenagers organizing on TikTok reserved more than a million tickets for a Trump rally in Tulsa, Okla., that Mr. Parscale had organized, inflating the numbers as a prank. Only about 6,200 people showed up, infuriating the president.

At the same time, Mr. Parscale’s spending decisions were increasingly being questioned; the campaign had blown through more than $1 billion since the beginning of 2019, and Mr. Trump still trailed in the polls. At the White House, Mr. Trump was livid about his standing in the polls. Mr. Kushner agreed that a change was needed and supported the decision to elevate Bill Stepien and demote Mr. Parscale.

When the end came, it was Mr. Kushner, not the president, who told him that he was being replaced, another blow to Mr. Parscale’s ego.

‘I Gave Every Inch’

While friends advised Mr. Parscale to make a clean break from the campaign, he chose instead to accept a smaller role. For the Republican National Convention, Mr. Parscale was in charge of video supplements to the program. Working mostly from his Florida home, he became frustrated.

In a recent interview on Fox News, Mr. Parscale blamed his enemies in Mr. Trump’s orbit (without naming them) for his downfall.

He told the Fox News anchor Martha MacCallum that he was no longer in touch with Mr. Trump. “It’s pretty hurtful,” he said. “But it’s probably just as much my fault as his. I love that family. And I gave every inch of my life to him, every inch.”

If the purpose of the interview was to ingratiate himself with the president or his family, it backfired. Mr. Kushner has told White house aides and other allies he thought it was a bad idea. And Mr. Trump, those people said, remains irritated that Mr. Parscale became rich and famous trading off his name.

When Mr. Stepien took over as campaign manager, there were discussions about reviewing spending decisions made under Mr. Parscale, but with only about three months left until the election, the decision was made to focus on reining in the budget going forward and not revisiting the past.

Mr. Parscale has denied using funds inappropriately and said the Trump family approved all his spending decisions.

Current and former Trump officials said they interpreted Mr. Parscale’s re-emergence on Fox News after two months of silence as an attempt to increase the value of the memoir he has talked about writing, and to ingratiate himself with a president who may end up retaining a good deal of influence over the Republican Party in the years ahead. He is also trying to rehabilitate his reputation to better promote his new company.

Of the police episode in September, Mr. Parscale said he had been breaking down from stress, anxious about attacks from his own side and still grieving the loss of twin children who died as newborns in 2016.

The promotional material for Nucleus is bare-bones, with a few bullet points of description. “A web-based digital infrastructure creates centralized hub for campaign,” one reads. He changed the Parscale Strategies site from a stark photo of his face and beard in profile to a more corporate-looking landing page advertising, “innovative marketing solutions.”

For now Mr. Parscale’s political legacy is that he was right about Facebook and that he helped Donald Trump score a stunning victory.

Today his campaign tactics — rapidly testing ads to see what gets clicks, pumping funding into Facebook rather than just television — seem obvious.

“It’s easier to think the bad ads brainwashed people and that Brad Parscale tricked them,” said Jessica Baldwin-Philippi, an associate professor at Fordham University who is writing a book called “Mythologizing the Data Campaign.” “If you have a dark ad about a migrant caravan but the candidate is also saying that, well, it’s not that secret and dark.”