Donald Trump fired campaign manager Corey Lewandowski on Monday, ousting the man who helped him to a stunning victory in the Republican presidential primaries as the billionaire businessman begins a general-election reboot.

Analysts said the firing could buy Mr. Trump time to appease Republican leaders who have been losing confidence in their party’s presumptive presidential nominee.

Campaign Chairman Paul Manafort, who reportedly has clashed with Mr. Lewandowski, will take over day-to-day operations in an effort to build the kind of machine needed to raise money, deploy staff and compete on the airwaves for a successful national campaign against presumptive Democratic nominee Hillary Clinton.

“Trump’s lagging in so many different areas — fundraising, field, communications, advertising — that something drastic needed to be done,” said Eric Fernstrom, a top adviser to Mitt Romney’s 2012 presidential campaign. “Making a change at the top has the potential to transition the campaign into a new phase where they start meeting all the usual metrics of a campaign at this stage of the race.”

The Trump campaign didn’t say why it made the change. Instead, it released a brief statement thanking Mr. Lewandowski, who signed up early last year.

“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,” campaign spokeswoman Hope Hicks said in a statement.

Tensions within the campaign were clear for months, with reports of Mr. Lewandowski and his allies battling Mr. Manafort, and both sides leaking dirt to reporters.

Indeed, after Mr. Lewandowski’s firing, Michael Caputo, a campaign aide, tweeted: “Ding dong the witch is dead!” Mr. Caputo later left the campaign and deleted references to his role in the campaign from his Twitter profile.

Mr. Lewandowski said on CNN that he doesn’t know why he was fired, but he praised Mr. Trump’s campaign accomplishments. “If Donald Trump wins, that’s good for Corey Lewandowski and it’s good for the country,” he said.

Mr. Lewandowksi seemed a good fit with Mr. Trump’s brash style but caused headaches for the campaign after an altercation with a reporter in Florida. Mr. Lewandowski initially denied touching Breitbart.com reporter Michelle Fields, but video showed he did.

Police investigated, but prosecutors did not pursue charges.

More recently, campaign analysts have questioned whether Mr. Lewandowski, whose experience includes running a Republican senator’s doomed re-election campaign, working as a lobbyist and helping a conservative activist group, could shoulder the responsibilities of a general election campaign.

The general consensus is that Mrs. Clinton has developed a well-oiled campaign operation and a much deeper and more reliable fundraising network compared with Mr. Trump, who boasted during the primary about self-funding his campaign and riding a lean campaign operation to victory.

John Brabender, a senior strategist with former Sen. Rick Santorum’s 2012 presidential campaign, said he stopped by Trump Tower a couple of weeks ago to say hello to friends and that the campaign headquarters seemed better fit for a congressional candidate than that of a presidential nominee.

“I think the biggest problem was that for all practical purposes, there were two campaign managers. Donald Trump was dating two campaign managers, and he has decided to get serious with one of them,” he said.

He said the move could help Mr. Trump buy time with donors who have been reluctant to open their wallets.

“There are a lot of particularly donor types that are sitting on their hands until they see something changed organizationally,” he said. “They might see this as a first step toward making this into a legitimate organized presidential campaign.”

Fred Malek, finance chairman of the Republican Governors Association, said it is hard to predict how a changing of the guard will influence the campaign until “we see who is added and whether Trump will take guidance from the new campaign leadership.”

“On the fundraising side, most donors are looking for Trump to be more inclusive and substance-oriented. However, even at that, it’s awfully difficult to mount a competitive fundraising effort on so short a time,” Mr. Malek said.

The concerns about Mr. Trump’s campaign have been further inflamed by his criticism of a federal judge overseeing a case involving Trump University, doubling down on a proposed temporary Muslim ban after the terrorist attack in Orlando, Florida, and his assertion that Republican leaders should “shut up” and “toughen up.”

A Monmouth University poll released Monday found Mrs. Clinton leading Mr. Trump among registered voters by 47 percent to 40 percent and that about half of those respondents said it is “very important” to make sure Mr. Trump does not win the election.

Democrats, meanwhile, mocked the idea that the face-lift will help the Trump campaign, saying “Corey Lewandowski is not the problem; it’s Donald Trump.”

“Today’s shuffling of staff does nothing to make up for Trump’s divisive and racist comments and policies,” said Mark Paustenbach, a spokesman for the Democratic National Committee. “It does reinforce, however, that there’s no one Trump won’t throw overboard for his own personal benefit, just as he’s done his entire life.”

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