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Donald Trump's campaign manager Corey Lewandowski, who has been with Trump since the beginning, has been ousted from the campaign.

The decision to part ways with the loyal aide was Trump's and he told Lewandowski Monday morning in a "direct conversation." Trump's top strategist, Paul Manafort, will be moved into the campaign manager role, two sources tell NBC News.

While Trump and Lewandowski are close, Trump made the decision because of systemic problems in the campaign that had spilled out into the public, leaving Trump's campaign floundering amid sinking poll numbers and image problems, multiple sources from inside the campaign said.

“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,” spokeswoman Hope Hicks said in a statement. “The campaign is grateful to Corey for his hard work and dedication and we wish him the best in the future.”

Speaking on Fox News Channel later Monday evening, Trump himself told Bill O'Reilly, "[Lewandowski’s] a talented person. He’s a good guy. He’s a friend of mine. But I think it’s time now for a different kind of a campaign."

Lewandowski's firing is a significant move that will have major implications on the campaign just one month before the Republican Party's nomination convention. It also appears to be a surprise as Lewandowski was with Trump at campaign events and fundraisers as recently as this weekend.

Lewandowski's exit comes at a time when Trump's poll numbers are slumping, he's struggling to implement an organization to run a national campaign, criticism from Republican officials continues to mount, and efforts to deny Trump the nomination emerge.

“I’m willing to share in the successes and also share in the failures. Being the head of the organization, if there are failures of this campaign they lay in my hands and that is what happens when you are a leader and there are consequences for failures,” Lewandowski said in an interview on MSNBC's "MTP Daily."

Though he declined to get into the specifics of why he was let go, Trump's former right-hand man did mention his boss' second-place finish in Iowa as an area he came up short. Texas Sen. Ted Cruz won the first nominating contest of the 2016 campaign, though Trump quickly rebounded with a major win in New Hampshire.

Related: The United States of Trump: An inside look at the voters who took over the Republican Party

After the primaries concluded, Lewandowski had been assigned internal management of major portfolios, including Republican outreach, the running mate search and liaison to the transition team. But a reservoir of mistrust existed in the fragile relationship between Lewandowski and the Republican National Committee. Lewandowski told associates he did not feel the RNC had been fully transparent in early meetings after Indiana's primary. There was concern about whether the party would fully get behind Trump or focus more on preserving a majority for Speaker Ryan and Senate Republicans, according to concerns.

Lewandowski remained loyal to his former boss. In an earlier interview with NBC News, Lewandowski said he is "honored" to have been able to work with Trump and that he will continue to support the nominee in his role as head of the New Hampshire delegate chair. He also offered to play a role in the campaign in either a "formal or informal" role.

The Trump campaign had been in internal turmoil as Lewandowski clashed with Manafort, a seasoned campaign operative who has attempted to polish Trump and employ a more traditional campaign structure. Lewandowski is known for letting "Trump be Trump," a mantra that some within the campaign say has caused serious, unnecessary errors. And the two have blocked each other from hiring top aides.

On Fox News, Trump acknowledged to O'Reilly that his campaign doesn't have an traditional structure.

"I have 73 people," Trump said. "Hillary Clinton has like almost 900 people. And we’re in the same position. So you know there’s something nice about that. I got criticized for that."

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Trump's children, especially Ivanka, have been unhappy with Lewandowski, according to campaign sources. She has been unnerved "for months," especially after Lewandowski was accused of physically assaulting female reporter, Michelle Fields. But Trump had remained loyal to his campaign manager. When asked if Trump would dump Lewandowski in March after, Trump said, "I don't discard people. I stay with people."

Much of Lewandowski's power was reinforced by his access to the candidate. He was almost always at Trump's side and able to deflect his own detractors.

That strategy has, until now, worked.

After Trump's drop in the polls, his children were able to make the case that Lewandowski was no longer fulfilling a need. Ivanka and his other children, Donald Jr. and Eric, attended the meeting Monday morning when Lewandowski was let go.

Sources say this move is intended to be a signal that Trump wants to win and knows he must act quickly. "It should give Republicans some comfort that he gets it," said a Trump insider.

"He knows that it hasn't been working," said another source.

For those in the GOP establishment and more broadly who have asked and even pleaded, "Is he willing to do what is necessary?" Lewandowski's firing is a signal that Trump is willing to do what it takes to win.

Highlighting the divisions within the campaign, Trump's New York state director Michael Caputo took to Twitter immediately after the news broke.

Caputo resigned hours after sending that tweet.

“I don’t know Michael Caputo and he was never paid by the campaign, he was a volunteer so he’s welcome to tweet anything he wants to,” Lewandowski said on MSNBC. “He’s an overzealous volunteer.”

Campaign adviser Tana Goertz, who like Lewandowski has been a part of Trump's campaign since he announced one year ago, said his firing "needed to be done."

"This was news to me, yes definitely, but the campaign has move into a new direction," Goertz said on MSNBC. "Going into the general election we all have the same goal and that is to get Mr. Trump into the White House and defeat Hillary Clinton. So this was-this was what needed to be done.

Here's a timeline of some of the major moments of Corey Lewandowski's tenure, beginning with Trump's New Hampshire win.

February 9, 2016: Trump wins New Hampshire primary.

March 8, 2016: Lewandowski allegedly yanks on the arm of reporter Michelle Fields at an event at Trump National Golf Club.

March 10, 2016: Fields publishes a full account of the incident on Breitbart’s web site. Trump’s campaign responds, calling the allegation “entirely false.” Fields tweets a photo of bruises on her arm.

March 11, 2016: Lewandowski tweets at Fields "you are totally delusional. I never touched you. As a matter of fact, I have never even met you.”

March 15, 2016: Despite the swirling controversy, Trump publicly congratulates Lewandowski at his March 15 primary victory speech, saying "good job, Corey."

March 19, 2016: Video captures Lewandowski grabbing a protestor’s collar at a rally in Tucson, Arizona.

March 21, 2016: Buzzfeed publishes a story detailing misogynistic comments by Lewandowski, particularly toward female reporters.

March 28, 2016: Trump hires strategist Paul Manafort to lead his delegate effort.

March 29, 2016: Lewandowski is charged with battery for the Fields incident by Jupiter police. Trump continues to defend Lewandowski, saying of Fields "How do you know those bruises weren’t there before?”

April 8, 2016: Manafort tells CNN he works "directly for the boss."

April 14, 2016: Officials in Palm Beach County drop assault charges against Lewandowski.

April 18, 2016: Reports begin to emerge that Lewandowski has essentially been reduced to a "body man and scheduler” as Manafort ascends to de facto campaign manager, although he retains the campaign manager title.

May 10, 2016: The Washington Post reports that Lewandowski will oversee Trump’s vice presidential search.

May 16, 2016: Lewandowski denies reports that he is writing a book about the campaign.

May 19, 2016: Paul Manafort is promoted to campaign chairman and chief strategist, prompting yet more speculation that Lewandowski is being pushed out entirely. Spokeswoman Hicks says Lewandowski will "continue overseeing day-to-day operations and will work with Manafort on political strategy and communications, among other things, through the general election.''

May 19, 2016: Page Six reports that Lewandowki and spokeswoman Hope Hicks had a public "screaming match" on a Manhattan street.

May 25, 2016: Trump aide Rick Wiley leaves campaign after six weeks on the job.

May 29, 2016: Lewandowski appears on Fox News Sunday and dismisses reporting indicating a rift between himself and Paul Manafort, saying “there is no sunlight between me and Paul.”