(Reuters) - Donald Trump’s U.S. 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 Nov. 8 election. Trump said in a statement he had accepted Manafort’s     resignation and praised his “great work,” 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.  Nonetheless, he appeared to stray at times from his prepared remarks, as when he said that black voters, who overwhelmingly tell pollsters they prefer Democratic candidates, should vote for him. “You’re living in poverty, your schools are no good, you have no jobs, 58 percent of your youth are unemployed,” Trump said. “What the hell do you have to lose?” Trump, who has never held elected office, did not refer to his staffing changes, but in an interview earlier on Friday his 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 the Fox News Channel’s “Sunday Morning Futures”, 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.  Federal investigators from the U.S. Department of Justice are examining American ties to corruption in Ukraine, CNN reported on Friday. The investigation is looking at the work of Manafort’s firm and another lobbying firm, the Podesta Group, headed by Tony Podesta, whose brother, John Podesta, is chairman of Clinton’s presidential campaign. Clinton leads Trump by 8 percentage points among likely voters, according to a Reuters/Ipsos opinion poll released on Friday. The Aug. 14-18 online poll showed that Clinton was supported by 42 percent of Americans who are expected to vote, compared with 34 percent for Trump. Some 23 percent would not side with either candidate.  On Wednesday, Trump overhauled his campaign team, hiring the head of conservative website Breitbart News, Stephen Bannon, as chief 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, who first joined the team in March, took over the running of the campaign from Corey Lewandowski, who was fired as campaign manager in June.     The Clinton campaign has pointed to Manafort’s Ukraine work and favorable comments that Trump has made about Russian President Vladimir Putin to sow questions in voters’ minds about whether the Russian government has an unseen hand in the U.S. 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. 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 any time he had caused “personal pain” with his take-no-prisoners style.  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 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 Sept. 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 U.S. federal judge ruled that Clinton did not need to give sworn testimony in a lawsuit about the email issue. 