Former Clinton campaign manager on Democratic debates: "This is just the beginning"

U.S. intelligence officials are in no doubt Russia tried to influence our 2016 election, and believe Moscow will likely do the same in 2020. Robby Mook was right in the middle of that 2016 election as Hillary Clinton's campaign manager. The Mueller report says Russia tried to hurt the Clinton campaign and benefit her opponent, Donald Trump. Mook, who is now the president of the House Majority PAC, joins "CBS This Morning" to discuss Robert Mueller's decision to testify publicly in front of Congress and what we should expect from the first round of Democratic presidential candidate debates Wednesday night.