If there is any obvious takeaway from this first tipping of Mr. Mueller's hand, it's that the veteran and highly respected federal prosecutor is engaged in a long-term and meticulous quest that Mr. Trump can short-circuit only at the risk of further imperiling his own case. He and his legal advisers have repeatedly said he has no intention of firing Mr. Mueller, an action that would only hand the investigating sleuths prima facie grounds for charging abuse of presidential power.