WASHINGTON — Rod J. Rosenstein, the deputy attorney general, has asked federal prosecutors to help review the government documents of Judge Brett M. Kavanaugh, President Trump’s Supreme Court nominee, according to a letter obtained by The New York Times on Wednesday. Mr. Rosenstein’s request was an unusual insertion of politics into federal law enforcement. While the Justice Department has helped work on previous Supreme Court nominations, department lawyers in Washington typically carry out that task, not prosecutors who pursue criminal investigations nationwide. But in an email sent this week to the nation’s 93 United States attorneys, Mr. Rosenstein asked each...