Special counsel Robert Mueller, who is investigating ties between Russia and President Trump’s presidential campaign, last week subpoenaed Steve Bannon to testify before a grand jury, a source told the New York Times.The subpoena calls for Bannon to testify about possible links between Trump and the Kremlin, but the Times points out that Mueller may be employing a negotiation tactic to get Bannon to agree to be questioned by investigators in a less formal setting at the special counsel offices in D.C.Bannon served as chief executive of Trump's presidential campaign in its final months and is often credited with orchestrating his unexpected victory more than a year ago.After the election, Bannon held a position as President Trump’s chief strategist until he left the White House in August and returned to Breitbart News. Bannon resigned from Breitbart a week ago. In recent weeks, the president has turned against Bannon and even given him the derogatory nickname “Sloppy Steve.” According to Michael Wolff’s “Fire and Fury,” Bannon called a meeting Donald Trump Jr. had with a Russian lawyer “treasonous.”