WASHINGTON (Reuters) - Congress is expected to approve President-elect Donald Trump’s choice of retired Marine Corps General James “Mad Dog” Mattis as secretary of defense, despite Democratic concerns that it ignores a long tradition of civilian control of the military. For Mattis to be confirmed, the Senate and the House of Representatives both must pass a waiver exempting him from a law written when the Department of Defense was created to ensure that the military is under civilian command.Legislators have granted such a waiver only once, in 1950, when Congress passed an act that allowed General George Marshall, who had retired in 1945, to serve as Pentagon chief.   The 66-year-old Mattis, who is revered by fellow Marines, retired 3-1/2 years ago. The 1947 National Security Act requires a seven-year gap between active duty military service and the Cabinet position, leaving the chairman of the Joint Chiefs of staff as the Pentagon’s top uniformed military position. Another Marine general, Joseph Dunford, is the current chairman, serving a two-year term that ends in mid-2017. Mattis was once Dunford’s commander. Lawrence Korb, who was an assistant secretary of defense under Republican President Ronald Reagan, said having a military officer as secretary of defense would rob the Pentagon of needed perspective. He noted, for example, that social change in the military - from ending segregation to allowing women in combat - has always been pushed by civilians. “I don’t think it’s a good idea to have a military person as secretary of defense,” he said Friday in a telephone interview. Trump takes office on Jan. 20. After Trump announced his selection on Thursday, Democrats joined many Republicans, who control majorities in both houses of Congress, to heap praise on Mattis. But a few also raised concerns about having too many generals in the top tiers of the government. Trump’s choice for national security adviser is Michael Flynn, a retired Army lieutenant general and former director of the Defense Intelligence Agency. The president-elect is also said to be considering David Petraeus, another retired Army general, as secretary of state, and retired Marine General John Kelly to head the Department of Homeland Security.  Senator Kirsten Gillibrand of New York, a Democrat on the Armed Services Committee, said on Thursday she would not back a waiver for Mattis.  Chris Murphy, another Senate Democrat, said on Friday he was “deeply fearful” that the precedent of civilian control of the military could wither, although he would talk to Mattis and spoke highly of the military leader. “It would be a really dangerous precedent to break here,” he told a news conference in Connecticut, his home state. Representative Adam Smith, the top Democrat on the House Armed Services Committee, had high praise for Mattis but said the House should perform a full review, including committee hearings, if it were to consider overriding the prohibition on recent military officers leading the Pentagon. “Civil control of the military is not something to be casually cast aside,” Smith said in a statement. 