Donald Trump's US presidential campaign sealed a major staff reshuffle with the resignation on Friday of its campaign chairman and the Republican nominee tried to end weeks of upheaval to focus on beating Democratic rival Hillary Clinton.
The departure of Paul Manafort came as Trump tried this week to reset his unorthodox bid for the White House after falling behind Clinton in opinion polls for the November 8 election.
Trump said in a statement he had accepted Manafort's resignation but did not offer an explanation for the departure.
Trump aides say the staff changes usher in a greater focus on policy and a more serious tone.
The former reality TV host has stuck to that mission, swapping his free-wheeling rally speeches for prepared remarks that stick to a singular theme. He has concentrated on immigration, trade deals and law and order.
Speaking in Dimondale, Michigan on Friday with the use of a teleprompter, Trump stayed clear of much of the hyperbole that has been a hallmark of his campaign.
He attacked Clinton for her position on trade agreements, telling the Michigan crowd that the Democrat would send auto jobs to Mexico.
"Michigan is going to end up with lots of empty buildings all over the place unless you elect Donald Trump president," he said.
Trump's son, Eric Trump, said unflattering headlines about Manafort had taken a toll.
"I think my father didn't want to be, you know, distracted by, you know, whatever things that, you know, Paul was dealing with," he told Fox News, while also praising Manafort's work for the campaign.
Questions have arisen about Manafort's previous work for the political party of the Kremlin-backed former Ukrainian leader, Viktor Yanukovich.
Trump, who has never held elected office, has also been embroiled in a series of controversies in recent weeks and has lagged Clinton in opinion polls.
CLINTON POLL LEAD
Clinton leads Trump by eight percentage points among likely voters, according to a Reuters/Ipsos opinion poll released on Friday. The August 14-18 online poll showed that Clinton was supported by 42 per cent of Americans who are expected to vote, compared with 34 per cent for Trump. Some 23 per cent would not side with either candidate.
On Wednesday, Trump overhauled his campaign team, hiring the head of conservative website Breitbart News, Stephen Bannon, aschief executive of the campaign in a move that bolstered his combative image. Trump also promoted adviser Kellyanne Conway, a data-driven political analyst, to campaign manager.
In a previous shake-up, Manafort took over the running of the campaign from Corey Lewandowski, who was fired as campaign manager in June.
A person close to the campaign said Trump had been unhappy with Manafort for a variety of reasons. Manafort, who first joined the campaign in March, had presided over a period in which Trump had formally sealed the Republican presidential nomination after seeing off 16 rivals.
But there were strains, the person said, describing how Trump became angry at reports that Manafort promised Republican officials that Trump's bombastic persona on the campaign trail was merely an act.
"This morning Paul Manafort offered, and I accepted, his resignation from the campaign," Trump said in a statement on Friday. "I am very appreciative for his great work in helping to get us where we are today."
Manafort has come under scrutiny over his work with pro-Russian political groups in Ukraine. Earlier on Friday, a Ukrainian lawmaker offered more details of what he said were records of cash payments allocated to Manafort by Yanukovich's party. Manafort, in a statement earlier this week, has denied any wrongdoing.
The Clinton campaign has pointed to Manafort's Ukraine work to add to its criticism of favorable comments that Trump has made about Russian President Vladimir Putin and to sow doubts in voters' minds about whether the Russian government has an unseen hand in the US election. Russian officials have rejected that accusation.
"You can get rid of Manafort, but that doesn't end the odd bromance Trump has with Putin," Robby Mook, Clinton's campaign manager, said in a statement.
'TOO MANY COOKS'
John Feehery, a Republican strategist, said it would have been unsustainable for Manafort to stay on after Wednesday's hires. "Too many cooks in the kitchen," he said.
Trump's hiring of Bannon was seen as a sign he would not hold back in his aggressive, unorthodox campaign manner. Still, Trump offered rare words of regret on Thursday for causing offence with his take-no- prisoners style.
Reading from a teleprompter, he told supporters in North Carolina that he had sometimes misspoken. "I regret it," he said, "particularly where it may have caused personal pain."
On Friday, he also began airing his first television advertisement since becoming the Republican candidate, buying airtime in the crucial states of Ohio, Pennsylvania, North Carolina and Florida, where Clinton has already spent of millions in television advertising.
The 30-second ad focuses on immigration, saying Clinton would allow "terrorists and dangerous criminals" into the country, a charge her campaign dismisses.
Karlyn Bowman, a senior fellow and research coordinator at the American Enterprise Institute conservative think tank, said Trump still had some time to turn around his campaign, noting the news of the shakeup came as many Americans were enjoying summer vacations.
The final stretch of the protracted campaign traditionally starts after Labor Day, which falls on September 5 this year.
"I'm not sure the public pays a lot of attention to inside campaign stories," she said. "But that said, Trump has clearly been having significant problems in the polls and he needed to do something differently and perhaps this is the beginning of the attempt."
Clinton won some relief on Friday from a controversy over her use of a private email system when she was secretary of state.
A US federal judge ruled that Clinton did not need to give sworn testimony in a lawsuit about the email issue.