NEW YORK – 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 8 November 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 US 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.