President Donald Trump on Monday directed former White House counsel Don McGahn to defy a subpoena by not testifying before Congress, citing a Justice Department memo arguing that lawmakers can't force his testimony about special counsel Robert Mueller's Russia investigation.McGahn had been subpoenaed to testify before the House Judiciary Committee on Tuesday at 10 a.m., but the DOJ has issued a legal opinion arguing that Congress can't compel him to testify. In a letter to House Judiciary Chairman Jerry Nadler, White House counsel Pat Cipollone wrote that Trump has told McGahn not to attend the Tuesday hearing, a directive he argues is "firmly rooted in the Constitution's separation of powers and protects the core functions of the Presidency.""The Department of Justice has provided a legal opinion stating that, based on long-standing, bipartisan, and Constitutional precedent, the former Counsel to the President cannot be forced to give such testimony, and Mr. McGahn has been directed to act accordingly," White House press secretary Sarah Sanders said in a statement. "This action has been taken in order to ensure that future Presidents can effectively execute the responsibilities of the Office of the Presidency."Democrats are eager for McGahn's testimony since he was frequently mentioned in Mueller's redacted report. In the second volume of the report, about potential obstruction of justice, the special counsel detailed Trump pushing McGahn to fire Mueller, which McGahn refused to do.In his report, Mueller found no conspiracy between Russia and Trump's 2016 presidential campaign. And while the special counsel couldn't conclude that Trump obstructed justice, the report said it also couldn't exonerate him.McGahn has previously defied a subpoena from the Judiciary Committee. Earlier this month, Trump instructed McGahn not to hand over documents he provided to Mueller during the investigation – a request McGahn honored. The White House later invoked executive privilege to prevent public access to the full, unredacted Mueller report.The White House's push for McGahn to defy the subpoena will continue to escalate sky-high tensions between the executive and legislative branches. Trump has signaled that he'll refuse to comply with all subpoenas from House Democrats looking into the president, his allies, his businesses and finances.Nadler, New York Democrat, has said he'll schedule a committee vote to hold McGahn in contempt if he doesn't appear on Tuesday.If McGahn doesn't appear, he will be the second individual affiliated with the administration to be a no-show before the House Judiciary Committee.Attorney General William Barr skipped his testimony about Mueller's report before the committee in early May, leading to a 10-minute, empty-seat hearing. The committee later voted along party lines to hold Barr in contempt.