A Ukrainian lawmaker releases more details of what he says were secret payments made to Paul Manafort, Donald Trump's former campaign chief who resigned the same day the new details emerged. (Reuters)

Donald Trump’s presidential campaign endured another day of internal disruption Friday when Paul Manafort, who as campaign chairman had sought to build bridges with the Republican establishment by encouraging a more temperate and consistent message from the GOP nominee, abruptly resigned after a staff shake-up that had reduced his role.

The decision came during a critical week in the campaign, with Trump under pressure from Republican leaders and wealthy contributors to re-balance his candidacy after weeks of missteps that have left him trailing Democratic nominee Hillary Clinton in national and battleground-state polls. The resignation also followed increased scrutiny of Manafort's earlier work in Ukraine on behalf of pro-Russian figures, including the country's former president, Viktor Yanukovych.

Manafort’s departure leaves the campaign more firmly under the guidance of newly installed chief executive Stephen K. Bannon, formerly of Breitbart News, and veteran pollster Kellyanne Conway, who was elevated to campaign manager earlier in the week.

Whether Manafort’s resignation will lead to more staff exits was not clear Friday, though Trump advisers said Bannon and Conway are considering additional hires. The longtime GOP strategist had recruited a number of former associates to join the campaign after he arrived in the spring.

The resignation was formally announced just as Trump was preparing to tour the devastation from extraordinary flooding in Louisiana. The candidate said in a statement: “This morning Paul Manafort offered, and I accepted, his resignation from the campaign. I am very appreciative for his great work in helping to get us where we are today, and in particular his work guiding us through the delegate and convention process. Paul is a true professional and I wish him the greatest success.”

The news followed a rally Thursday night in North Carolina in which the candidate expressed regret for some of the harsh language he has used during the campaign. Trump’s speech represented a sharp change in tone by a candidate who has resisted any such moves in the past. Trump’s words were applauded by mainstream Republicans, who hope this marked a transition to a campaign focused on attacking Clinton and delivering a consistent message about national security, law and order, and the economy.

Friends of Manafort said Friday they thought he was undone by the combination of revelations about his work on behalf of pro-Russian figures and the elevation of Conway and Bannon to Trump’s new inner circle.

“If you had had one of these things happen, it would have been survivable. But you had two of these things in concert,” said a GOP strategist who knows Manafort and, like others interviewed, requested anonymity to comment on the Trump campaign. “One thing I don’t think Trump will tolerate is the focus being on someone else rather than himself.”

Eventually, Manafort’s background caught up with him. According to two people familiar with Trump’s decision, Trump on Thursday night was given a copy of an Associated Press story about how Manafort’s firm had not properly disclosed its foreign lobbying, shortly before taking the stage in North Carolina. Trump “blew a gasket,” one person said, and told Bannon and others that he should be dismissed.

Manafort was then told in candid terms about Trump's view, and he prepared to resign. These exchanges were first reported by the New York Times.

“He was asked and he indeed tendered his resignation today,” Conway said Friday on WABC radio.

Associates of Manafort said Friday that it was clear that he was taking a calculated risk by joining Trump’s campaign. “He knows he’s been doing this stuff," the GOP strategist said. "It was going to become an issue. He wasn’t prepared to tamp it down. When he decided to re-enter high-profile American politics and he ratcheted it up with lots of Sunday shows and TV appearances, he had to know he was putting himself out there as a target.”

Members of Trump’s family, which has been an important part his inner circle, praised Manafort after the announcement but made clear that controversies around him played a role in his decision to leave.

“I think my father didn’t want to be, you know, distracted [with] whatever things” Manafort was dealing with, said Eric Trump, Trump’s son, speaking to Fox News. “My father just didn’t want to have the distraction looming over the campaign and quite frankly looming over all the issues that Hillary is facing right now.”

Shortly after the interview, Donald Trump tweeted to his son, "Great job @EricTrump."

Jared Kushner, husband of Donald Trump's daughter Ivanka Trump, had also become less enthusiastic about Manafort’s management, according to Donald Trump’s aides. Kushner gave his blessing Tuesday to the campaign changes and has told Trump privately that Manafort had difficulties with aspects of executing the Republican National Convention and with building Trump’s organization nationally, the aides said.

Coupled with Trump’s own frustrations, the family’s souring on Manafort’s leadership created an atmosphere that was less supportive, though not hostile, of the operative, the aides said.

When Trump met with family and friends Sunday at Trump National Golf Club in Bedminster, N.J., he fumed about news reports and leaks following a Saturday fundraiser in the Hamptons where donors voiced concern about Manafort and the campaign. When he reached out to Bannon and Conway about elevated roles, his family did not intervene, the aides said.

"Trump talked to a lot of people, looked around and said, 'We're not performing at the level we need to,' " said former House speaker Newt Gingrich, a Trump counselor. "He had been thinking about this for some time. It wasn't anti-Manafort. He knew the Mercers," a wealthy GOP family, "really liked Bannon, and Kellyanne has grown pretty close to Ivanka and was seen as someone very effective with messaging."

Trump’s campaign team has been in a near-constant state of upheaval since the spring, when Manafort joined the operation, as its leaders struggled to build out what had been an extremely lean operation during the primaries. Manafort clashed immediately with then-campaign manager Corey Lewandowski and eventually prevailed in a power struggle that led to the exit of Lewandowski, who played a central role in helping the novice politician win the Republican primaries.

Manafort was initially tasked with developing a strategy to assure Trump nailed down the number of delegates needed for a first-ballot victory at the Republican convention in Cleveland and was in charge of helping to plan the convention program.

On May 19, as Trump was nearing the necessary number of delegates to lock up the nomination, Trump promoted Manafort to the position of campaign chairman, a clear slight to Lewandowski. One month after that, Trump fired Lewandowski, clearing Manafort to run the campaign.

Manafort had repeatedly signaled to members of the Republican establishment that he and Trump were working together to re-brand the candidate in a more presidential light. But hopes of a “pivot” vanished as the summer wore on and Trump appeared intent on settling scores with critics and with former rivals within his party.

Perhaps most damaging was Trump's attack on a Gold Star family — who lost their son, Army Capt. Humayun Khan, while he served in Iraq — for speaking out against him at the Democratic National Convention. He later refused to apologize or express regret, although he did say late Thursday that he was sorry if he had caused "personal pain" for unspecified comments.

But aside from Trump’s missteps, Republican strategists also became increasingly concerned that the campaign, under the direction of Manafort, had failed to build out a robust infrastructure in key battleground states. Though the campaign is relying heavily on the Republican National Committee for its ground game, limited efforts by the main campaign have left Trump underprepared for the competitive general election, in the view of a number of GOP strategists.

Lewandowski, now a paid consultant to CNN, focused on that problem in an appearance on the cable network soon after Manafort’s resignation was public. “It’s been widely reported that [there] has not been a robust ground effort in states like Florida, that that had not been laid out yet,” he said. “You cannot blame the candidate for those things. Those things fall squarely on the staff at some level of building up the field teams and hiring the people."

Still, to some Republicans outside the Trump operation, Manafort’s departure will leave a void in the campaign. “Any semblance of the sort of structured political advice he was going to get from Manafort is going to be gone now,” said the strategist who knows Manafort. “You don’t have a voice in those meetings any longer that has presidential experience.”

Manafort is leaving on good terms with the campaign and will remain an ally and outside confidant of the operation, according to a close associate of his. Manafort had an easy rapport with Conway and Bannon in meetings this week but was inclined to leave to give them room to develop and execute their own strategy, aides said.

Rick Gates, who has been Manafort's influential deputy, will stay on and be based in Washington, according to Trump campaign spokesman Jason Miller, "taking over as the campaign's liaison" to the RNC.

Manafort’s personal business dealings have come under scrutiny in recent weeks, amid damaging questions over his ties to foreign governments and indications that he might have received $12.7 million in undisclosed cash payments. The alleged payments, which Manafort denied, were noted in a ledger kept by Yanukovych's political party. Since then, more evidence has surfaced that raised concerns about his ties to pro-Russian politicians in Ukraine.

 Jenna Johnson and Karen Tumulty contributed to this report.