WASHINGTON — Robert Mueller has been busy. In just the last two weeks, the special counsel has secured the cooperation of President Donald Trump’s former campaign chairman, dispensed with the case of the campaign aide who triggered the Russia probe and signaled he’s squeezed all the information he needs out of former national security adviser Michael Flynn. All that activity has not gone unnoticed by Trump. The president has continued to wage a public-relations war on Mueller, casting his investigation into Russian election interference and possible coordination with Trump associates as a politically motivated witch hunt.