Presidential historian Michael Beschloss explains the significance of yesterday's Bush-Obama attack on Trump

Appearing on MSNBC's Morning Joe, presidential historian Michael Beschloss marveled at former Presidents George W. Bush and Barack Obama publicly denouncing President Donald Trump -- albeit without mentioning his name-- saying it is a huge departure in presidential decorum.

Host Joe Scarborough suggested that the two ex-presidents -- along with Sen. John McCain (R-AZ) earlier in the week -- have no choice but to go after the "heart of Trumpism" that promotes bigotry.

"Michael, you had a very good and unusual scene yesterday," Scarborough said. "Many former presidents, especially if their last names are Bush. don't even talk about their successors in a negative way. George W. Bush for the most part bit his tongue for eight years. But George W. Bush, yesterday you had President Bush -- a member of the same party of the sitting president -- and Barack Obama, obviously attacking the sitting president on the same day on his brand of politics."

"George W. Bush debating bigotry and lies and isolationism," the MSNBC host continued. "This speech was aimed directly at the heart of Trumpism."

"Totally," Beschloss agreed. "And, Joe, it was unusual, you're right, the Bushes don't like to do this. George Bush 41 wrote Bill Clinton a letter on Inaugural Day in 1993, saying 'I will leave you alone --- don't have to worry about my criticizing you.'"

"The thing is, if you look at all this, who would have ever thought that in the year 2017 any former president would feel the need to criticize the sitting president of the United States?" the presidential historian reminded the MNSBC host. "I agree with you this was absolutely aimed at Donald Trump for encouraging bigotry and white supremacy. It's that that's not normal. It's that that should cause the big departure from tradition where modern presidents restrain themselves."

"Think of Jimmy Carter in the early 80's, when Ronald Reagan was following policies that were in many ways almost dianmetrically the opposite," he explained. "Carter was very quiet -- that's the norm. But for George W. Bush especially to depart from that tradition, you can imagine how strongly he feels."

Watch the video below via MSNBC: