WASHINGTON — President Trump has embarked on an aggressive new drive to rewrite the narrative of the Russia investigation by making dark and unsubstantiated accusations that former President Barack Obama masterminded a sinister plot to bring him down. On Twitter, on television, in the Rose Garden and even on an official White House social media page, Mr. Trump in recent days has taken aim at his most recent predecessor in a way that no sitting president has in modern times, accusing Mr. Obama of undefined and unspecified crimes under the vague but politically charged catchphrase “Obamagate.” The president went even...