Republican front-runner Donald Trump leaves his polling location in New York. In recent days, the presidential candidate has brought in advisers with extensive experience in campaigning. (Jabin Botsford/The Washington Post)

Donald Trump’s presidential campaign has undergone a series of sudden and dramatic changes in recent days, marking a key inflection point — a moment that either rescues his White House bid or one that came too little and too late.

Although Trump has won more than 20 state nominating contests — more than twice as many as his top rival, Sen. Ted Cruz (Tex.) — he is at risk of failing to gain the support of a majority of state-level delegates before the Republican Party’s convention in July.

The threat of coming so close yet still losing has prompted Trump to quickly rebuild his inner circle of trusted advisers, spend millions of dollars more on his operation and shift his campaign approach. He has brought in a team of seasoned Republican operatives while slowly diminishing the role of the less-experienced staffers who got him this far by simply allowing Trump to be himself.

Campaign manager Corey Lewandowski, 42, has largely been sidelined and is now functioning more like a scheduler than chief campaign architect. Meanwhile, Paul Manafort — a 67-year-old operative who has worked for several presidents and joined Trump’s team barely a month ago — has become Trump’s right-hand man and is swiftly filling the staff with his own associates.

The shift hasn’t been easy for many of the longtime staffers who believed in Trump when no one else did, made do with limited resources and tried their best to channel the immense energy behind his candidacy into voting booths. But that’s not enough anymore, prompting Trump to bring on staffers particularly focused on the behind-the-scenes battle for delegates.

Republican presidential candidate Donald Trump has hired more experienced political operatives and shuffled his top staffers as talk of a contested convention grows. One of his top staffers, national field director Stuart Jolly, resigned amid the changes. (Jenny Starrs/The Washington Post)

“When you bring other people in, I could see some people, their feelings get a little bit hurt,” Trump said in an interview Tuesday on the Fox News Channel, adding that everyone on his staff is getting along. “But, frankly, you know, we’re in a position where we’d like to see if we can close it out.”

Trump himself has changed. He seems to have heeded advice from his wife and daughters to tone down his rhetoric, although glimmers of his uncontrollable nature still shine through. He is tweeting less, skipping the Sunday news shows where pointed questions have recently tripped him up, reading from notes at rallies and refocusing on the economic issues that first brought him success early in the campaign. There are plans for him to soon give a series of policy speeches, perhaps with the assistance of a teleprompter — a device that to him once symbolized the bloodless establishment.

Since launching his once-long-shot campaign last June, Trump surrounded himself with fiercely loyal staffers who rarely challenged him. There’s Lewandowski, a gruff former law enforcement officer who had never worked on a presidential campaign. Michael Glassner, a former aide to Sarah Palin, was considered the strategic brains behind the campaign. Press secretary Hope Hicks is a 20-something who has had to learn about presidential politics on the fly, while Daniel Scavino Jr., a caddie-turned-executive, handles Trump’s social media.

The small crew traveled nearly everywhere with Trump on his jet, often bunking at his luxurious properties and private club in Palm Beach, Fla. They operated under a simple philosophy: Let Trump be Trump. Together the campaign largely ignored those who deemed the front-runner unpresidential: What he was doing was working, so why stop?

Lewandowski recruited a number of his former colleagues and friends to work for the campaign, and he delighted in turning down well-known strategists and GOP insiders.

But these aides are finding themselves overshadowed by the newly arriving strategists, members of an insider class that Trump used to mock on the trail.

Speaking with reporters Tuesday night ahead of a Trump news conference, Lewandowski shrugged when asked if he is okay with the campaign restructuring. “We’re growing, baby,” he said.

Trump’s inner circle is now dominated by Manafort, who long has had an apartment at Trump Tower, and a growing number of new hires. Rick Wiley, who has deep ties to the Republican National Committee, previously managed Scott Walker’s campaign and is now Trump’s national political director. William McGinley, a veteran GOP elections lawyer, will advise the campaign on delegate and convention rules.

Assisting with the effort to preserve and grow Trump’s delegate count are two former members of retired neurosurgeon Ben Carson’s presidential campaign, Ed Brookover and Barry Bennett.

The new team is expected to not only help Trump gain the nomination but also to build a campaign structure that could challenge likely Democratic nominee Hillary Clinton and heal Trump’s fractured relationship with the party.

Trump’s son-in-law, Jared Kushner, has also become a key policy adviser, especially on the Israeli-Palestinian conflict. Trump credited Kushner with helping to write a speech he gave at a pro-Israel conference last month, which members of Trump’s campaign team have pointed to as a blueprint for future policy speeches that could help shore up the candidate’s credibility. Kushner, who is married to Trump’s daughter Ivanka, is owner of the New York Observer, which recently endorsed Trump’s presidential bid.

The clearest sign of a new way of doing things came Monday, when Trump’s national field director, Stuart Jolly, sent him a resignation letter that began “Dear Mr. Trump” and often read more like a fan letter than a formal notice.

Jolly, 52, had never worked on a national campaign when he joined the staff — mostly to help his friend Lewandowski, whom he knew from their time working together at Americans for Prosperity, an advocacy group that is backed by a network of conservative donors organized by brothers David and Charles Koch.

While Lewandowski has been known to be tough on his staff — sometimes yelling or speaking harshly — Jolly was considered his friendly, soothing counterpart. But Jolly often lacked the knowledge of how to properly organize in key early-voting states, irritating several staffers at the state level.

Jolly said in an interview Monday that he recently learned that he would report to Wiley instead of Lewandowski. That’s when he decided it was time for him to leave.

“That wasn’t going to happen,” Jolly said of the change in bosses. “I just decided not to go that route and let the new team have a shot at it. We have a great group of folks in place on the ground, and they’re kicking butt and doing great work.”

Jolly included some advice to Trump in his resignation letter than echoes a concern held by several longtime staffers: “We did these simple things, we won, and we continued to win because we followed a winning formula. . . . My hope is that you will continue to listen to those who have propelled you to victory.”

Jose A. DelReal contributed to this report.