Special Counsel Robert Mueller is trying to head off the Paul Manafort defense team from questioning his prosecutors over press leaks. Mr. Mueller filed papers in U.S. District Court in Virginia opposing a request from Manafort attorney Kevin Downing to hold an investigative hearing into the rash of press reports on Mr. Manafort, President Trump’s former campaign manager. Mr. Downing cited 10 stories, sourced to unnamed people who implicated Mr. Manafort in possible criminal activity. He contends the information came from grand jury testimony, which would be illegal for a prosecutor to leak. In a footnote to his filing, Mr....