It was a sudden announcement made late Wednesday, one that signaled a sharp change of direction for the Trump campaign: Bill Stepien, a longtime political operative and deputy campaign manager for President Trump, would succeed Brad Parscale as campaign manager, as the president’s re-election effort struggles to gain its footing.

In Mr. Stepien, Mr. Trump has opted for a more traditional, professional operative than Mr. Parscale. Mr. Stepien is highly disciplined and data obsessed. He has been described as both shrewd and ruthless, a Machiavelli guided by vote-share spreadsheets.

Mr. Parscale at times seemed to seek the spotlight and even spent campaign money promoting his own Facebook page; Mr. Stepien is largely his opposite, preferring to operate completely behind the scenes.

“Stepien will be upset that I’m even speaking to you,” former Gov. Chris Christie of New Jersey, whom Mr. Stepien used to work for, said in an interview. “He is a guy who is, in my experience with him, allergic to press attention, or public attention of any kind really.”

He is also no stranger to rough-and-tumble politics.

Mr. Christie fired him over the so-called Bridgegate scandal, in 2013, which embroiled the governor’s administration after it came to light that officials had closed down access lanes to the George Washington Bridge to punish a political opponent. While Mr. Stepien denied involvement and was never criminally charged, there were indications that his hands weren’t entirely clean.

An aide to Mr. Christie testified that he told Mr. Stepien well before the scheme was hatched that the lanes at Fort Lee, on the New Jersey side, could be used to pressure Mark Sokolich, the town’s Democratic mayor, whose endorsement Mr. Christie sought for his re-election campaign.

Mr. Stepien’s name came up repeatedly during the trial, in testimony, texts and emails, and an internal report found that he had a romantic relationship with Bridget Anne Kelly, Mr. Christie’s former deputy chief of staff who wrote the infamous “time for some traffic problems in Fort Lee” message.

Though he maintained his innocence, Mr. Stepien declined to cooperate with federal and state investigators — at one point invoking his Fifth Amendment right against self-incrimination rather than comply with a legislative subpoena.

This year, the Supreme Court overturned the convictions of two other associates of Mr. Christie’s in the scandal, ruling that it was an abuse of power but not a federal crime.

In firing Mr. Stepien, Mr. Christie cited a “tone and behavior and attitude of callous indifference” in Mr. Stepien’s emails that “made me lose my confidence in Bill’s judgment.”

But the two have since reconciled.

“Even during that period of time, when then-candidate Trump came to me and asked me in ’16 about whether or not he should hire Bill, I told him he absolutely should,” said Mr. Christie, who said he now talked with Mr. Stepien multiple times a week. “There was not any falling out in terms of my trust in Bill’s skills during that period of time.”

Mr. Stepien, 42, got his start in politics in 1997, when he was still a student at Rutgers, volunteering for the campaign of State Senator Anthony R. Bucco of New Jersey. Mr. Stepien would work overnight in the headquarters, coming after hockey practice to send out mailers to absentee voters, reminding them to return their ballots on time.

From there, Mr. Stepien slowly climbed the ranks of New Jersey politics. He served as the driver for a Senate run by Bob Franks in 2000, as a lower-level staff member for Mr. Franks’s race for governor in 2001, and as the campaign manager for Bill Baroni’s State Assembly race in 2003, in which Mr. Baroni beat a Democratic incumbent in a traditionally Democratic district.

The Baroni victory put Mr. Stepien on the map in New Jersey. Mike DuHaime, a longtime New Jersey political operative, brought him into his consulting firm, a job that led to positions at the Republican National Committee and on the 2008 presidential campaigns of Rudolph W. Giuliani and Senator John McCain.

But his political breakout came with Mr. Christie in 2009, when Mr. Stepien managed his first campaign for governor. Though their opponent, Jon Corzine, was a vulnerable incumbent, Mr. Christie was also facing an independent candidate, Chris Daggett, whom internal campaign polls had at nearly 20 percent, keeping Mr. Christie stuck at 40 percent.

Mr. Stepien, along with Mr. DuHaime and Mr. Christie, hired TargetPoint, a Republican data firm, to ask Mr. Daggett’s supporters whom their second choice was.

“The ones who would be a Christie second choice, we just crushed them with mail and digital ads” against Mr. Daggett, Mr. DuHaime said. “We said this is a guy who wants to raise taxes and wants to raise tolls and gave them all the ideas about why he was further left than they thought. The Corzine second choice we ignored.”

Mr. Christie went on to beat Mr. Corzine by less than four percentage points.

In 2016, Jared Kushner, Mr. Trump’s son-in-law, hired Mr. Stepien onto the Trump campaign. Mr. Stepien toiled in the campaign war room, largely out of the spotlight, setting up field operations in battleground states. After the 2016 victory, Mr. Stepien joined the White House as the political director.

He has since been a key political figure inside Mr. Trump’s orbit, and was instrumental in persuading another New Jersey politician, Representative Jeff Van Drew, to switch his party registration from Democratic to Republican.

“He’s focused, hard-working,” Mr. Van Drew said in an interview. “I think he’s someone who is capable of having an open and honest dialogue with the president.”

But Mr. Stepien’s national profile grew as a result of his associations with Mr. Trump.

A few months after leaving his post at the White House in 2018, Mr. Stepien joined with Justin R. Clark, a lawyer who left the White House for the Trump campaign around the same time, to form a political consulting firm called National Public Affairs, according to corporate filings.

Since then, the firm has been paid more than $650,000 by a number of down-ballot campaigns, according to campaign finance filings. Mr. Stepien and Mr. Clark continued their outside political consulting through at least the end of last month.

Mr. Stepien and Mr. Clark did not respond to questions about whether the president’s campaign had a policy on outside work, or whether they planned to continue doing it.

Some of their clients are aligned with Mr. Trump’s politics, even if they’re not traditional Republicans, such as Gov. Jim Justice of West Virginia and Mr. Van Drew. But other clients left some national Republicans scratching their heads.

Mr. Stepien and Mr. Clark’s firm was paid more than $154,000 by the largely self-financed congressional campaign of Kathaleen Wall, a Republican Party donor in Houston who lost a bitter House primary runoff on Tuesday, a race that some in the party fear will leave their hold on the seat vulnerable in November.

Still, several Republican operatives said that Mr. Stepien’s promotion to campaign manager was a welcome change.

Neil Newhouse, co-founder of Public Opinion Strategies, a polling firm known for its work with Republican candidates, called Mr. Stepien a “no drama kind of guy,” adding that the shift signaled that the Trump campaign realizes it is in trouble.

“They’ve hit the emergency flasher in the campaign,” Mr. Newhouse said. “It’s an indication that they realize exactly where they are, they realize the urgent need to turn this around, and they’re sending out signals that that’s what they plan to do.”

The abrupt, late-night announcement from Mr. Trump caught many in the campaign by surprise, and some were still processing the transition in an emotional meeting on Thursday morning. A few hours later, Mr. Stepien issued a statement praising Mr. Parscale and Mr. Trump for the infrastructure they had built.

“President Trump and Brad Parscale have built an unprecedentedly strong campaign based on data and technology and Brad will remain heavily involved,” Mr. Stepien said. “We have a better team, better voter information, a better ground game, better fund-raising, and most importantly, a better candidate with a better record. With 109 days left, our goal is clear — to win each day we have left until Election Day.”

Mr. Stepien is moving fast: Even before his promotion was announced, he had already started his personal audit of the campaign, with a current focus on budgets and structure.

“With Stepien in charge, nothing will fall through the cracks, he will not miss a detail, and he will clearly make the campaign better,” Mr. Christie said. “But in the end, every campaign is more about the candidate than it’s about anybody else.”

Kenneth P. Vogel contributed reporting.