NEW YORK — Donald Trump abruptly fired campaign manager Corey Lewandowski on Monday in a dramatic shake-up designed to calm panicked Republican leaders and end an internal power struggle plaguing the billionaire businessman's unconventional White House bid.

In dismissing his longtime campaign chief — just a month before the party's national convention — Trump signaled, at least for a day, a departure from the seat-of-the-pants style that has fueled his unlikely rise in Republican politics. Perhaps more than anyone else in Trump's inner circle, the ousted aide has preached a simple mantra: “Let Trump be Trump.”

“I have no regrets,” Lewandowski told CNN just hours after he was escorted out of Trump's Manhattan campaign headquarters. Still, the former conservative activist seemed to acknowledge the limitations of his approach, which has sparked widespread concern among the GOP's top donors, operatives, elected officials and even some of Trump's family members.

“The campaign needs to continue to grow to be successful,” he said.

Trump, the presumptive GOP presidential nominee, described Lewandowski as a “good man” who helped “a small, beautiful, well-unified campaign” during the primary season.

“I think it's time now for a different kind of a campaign,” Trump said on Fox News Channel's “The O'Reilly Factor.”

People close to Trump, including adult children Ivanka, Eric and Donald Jr., had long-simmering concerns about Lewandowski, who had limited experience on the national scale before becoming Trump's campaign leader. Like many Republican officials, Trump's family urged him to professionalize a bare-bones campaign that had previously resisted adding staff and paid advertising heading into the general election.

A person close to Trump said Lewandowski was forced out largely because of the campaign's worsening relationship with the Republican National Committee, donors and GOP officials, who have increasingly criticized the candidate's message and campaign infrastructure in recent weeks. That person spoke on the condition of anonymity because that person was not authorized to discuss internal deliberations.

While Trump dismissed his critics publicly, he has been privately concerned that so many party leaders — House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell among them — have been reluctant to support him, the person said. Trump at least partially blamed Lewandowski.

“Firing your campaign manager in June is never a good thing,” said veteran Republican operative Kevin Madden. “The campaign will have to show dramatic changes immediately on everything from fundraising and organizing to candidate performance and discipline in order to demonstrate there's been a course correction. Otherwise, it's just cosmetics.”

Lewandowski's chief internal rival, campaign chairman Paul Manafort, largely inherits the campaign reins. The political veteran has long advocated a more scripted approach backed by a larger and more professional campaign apparatus, although Trump has shown little willingness to embrace a wholesale change in his approach.