Special counsel Robert Mueller and his prosecutors have invoked an unusual “conspiracy to defraud the government” charge to ensnare a Russian cyber network and could use the same legal strategy to go after President Trump and his associates, even if the conspiracy is not linked to a criminal act. Although the president constantly denies any wrongdoing, Mr. Mueller’s 10-month long investigation has become the heart of the Trump-Russian election meddling saga, dominating headlines and leaving the White House tangled in conflict and controversy as additional charges have also been filed against four of his close associates — with further indictments...