A former top aide to Donald Trump's presidential campaign will plead guilty to fraud-related charges within days – and has made clear to prosecutors that he would testify against Paul J. Manafort Jr., the lawyer-lobbyist who once managed the campaign. The change of heart by Trump's former deputy campaign manager, Richard W. Gates III, who had pleaded not guilty after being indicted in October on charges similar to Manafort's, was described in interviews by people familiar with the case. Mueller is heading the prosecutions of Gates and Manafort as part of the wide-ranging investigation into Russian meddling in the 2016 election and whether Trump or his aides committed crimes before, during or since the campaign.