A former Trump campaign aide who took a guilty plea in special counsel Robert Mueller’s Russia probe was the linchpin of the FBI’s investigation of Kremlin influence in the 2016 election, new reports say. In a drunken gossip session with an Australian diplomat in London in May 2016, Trump aide George Papadopoulos claimed that Russia held political dirt on Hillary Clinton, The New York Times said Saturday. Months later, e-mails hacked from the Democratic National Committee showed up on WikiLeaks, and the Australians told the FBI about the Papadopoulos conversation. The disclosure sparked the FBI’s probe into possible collusion between...