"It's long been said, President Reagan said it quite well, that America is the one place on earth that is not about a birthright. It's about the ascent to an idea," Meacham said.
"An idea that was written in one of the most important sentences ever in the English language, that all men were created equal and endowed by their creator with certain inalienable rights.
"We didn't apply that fully then, we haven't applied it fully now. But that journey towards a more perfect union is the story of the country.
"And what the president's done here is, yet again, because he did it after Charlottesville and I think he did it, frankly, when he was pushing the birther lie about President Obama, is he has joined Andrew Johnson as the most racist president in American history."
Meacham cited Johnson's state message that "African-Americans were incapable of self-government and relapsed into barbarism if they weren't closely supervized."
Johnson, a Democrat, became president in 1865 after the assassination of President Abraham Lincoln, to whom he was vice-president.
He opposed efforts to extend civil rights to black people, including the 14th Amendment, which granted citizenship to former slaves.
In 1866, Johnson vetoed the Civil Rights Act, but was overridden by Congress and it became law, making him the first president to which this happened.
Two years later, the House impeached Johnson—another presidential first—for making appointments to key roles without Congressional approval.
The Senate fell short by one vote of the two-thirds required to convict Johnson. A year later, in 1869, Johnson left the presidency.
Meacham pushed back against the criticism of Trump that his remarks are un-American, saying the country must take responsibility for the president it elected.
"When people say, 'This isn't who we are,' that's not true. It is who we are. It's who we are on our worst day and it's pointless to try to expiate ourselves from what Trump has been saying," Meacham said.
"There is a complicity, there is a national complicity in this, and the way America moves forward is 51 percent of the time we're with Lincoln instead of Andrew Johnson."