Donald Trump split Monday with campaign manager Corey Lewandowski, in arguably the highest-profile staff shake-up of the 2016 season – a move that comes after a tumultuous period for the billionaire businessman, and just weeks before he is set to claim the Republican presidential nomination at the GOP convention.

Senior Trump campaign aides confirmed to Fox News on Monday morning that Lewandowski is no longer with the campaign.

A campaign source told Fox News it was Trump's children who convinced the candidate to dump Lewandowski, amid an apparent power struggle between him and campaign chairman Paul Manafort. In that dispute, Trump sided with Manafort, who will now take over as campaign manager.

The departure of one of Trump's most senior and loyal advisers follows what was widely seen as a rough patch for the campaign.

Trump's polling numbers against presumptive Democratic nominee Hillary Clinton have slipped in recent weeks, and Trump has found himself caught up in several controversies over his own comments – in turn stoking tensions with other top Republicans.

Trump senior adviser Barry Bennett on Monday downplayed the shift in the polls, but suggested Trump would be talking more about the issues in the weeks ahead. Speaking with Fox News just minutes after the Lewandowski decision was reported, Bennett confirmed that Manafort is now running the operation.

“He’s the campaign chairman, so yeah – he’s the person in charge,” he said.

On Capitol Hill, a Republican congressional aide close to pro-Trump lawmakers said the campaign did not inform members of Congress of the Lewandowski departure. Still, some sources said they are not surprised by the move and had been given the impression there would be a pivot soon to the general election, involving a potential staffing change.

A hard-charging figure, and in some ways as unconventional as the candidate himself, Lewandowski had been by Trump's side since the beginning of his unlikely rise to presumptive GOP nominee. His main campaign experience before this cycle was managing the 2002 re-election campaign for then-New Hampshire Sen. Bob Smith; he subsequently worked for Americans for Prosperity.

Lewandowski has long been a controversial figure in Trump's campaign, but benefited from his proximity to the presumptive Republican nominee. He traveled with Trump on his private plane to nearly every campaign stop, giving him more direct access to the businessman than nearly any other campaign staffer.

He was a chief promoter of the idea that the best campaign strategy was to "Let Trump be Trump." He frequently dismissed the notion that Trump needed to hire more experienced political hands, spend on polling and sophisticated data operations, or moderate his rhetoric as he moved toward the general election.

Spokeswoman Hope Hicks said in a statement: "The Donald J. Trump Campaign for President, which has set a historic record in the Republican Primary having received almost 14 million votes, has today announced that Corey Lewandowski will no longer be working with the campaign. The campaign is grateful to Corey for his hard work and dedication and we wish him the best in the future."

The New York Times first reported on the Lewandowski development.

The move came as Trump faces continued deep resistance from many quarters of his party concerned by his contentious statements.

Aside from the controversies that have hung over the Trump campaign in recent weeks – including his comments claiming a judge is biased against him in the Trump University case because of his Mexican heritage; his renewed calls for a Muslim immigration ban; and his decision to revoke credentials for The Washington Post – the campaign is also entering a new and arguably tougher phase as Democratic groups start spending big against him.

Clinton and her Democratic allies have invested at least $41 million in commercials in crucial states such as Ohio, Florida and Nevada over the next six weeks. Those messages will be echoed by hundreds of Clinton workers in those same states and amplified by President Obama and other top Democrats.

Vice President Biden was also delivering an anti-Trump speech Monday.

FoxNews.com's Christopher Snyder and Fox News' John Roberts and Chad Pergram and The Associated Press contributed to this report.