DALLAS (CBSDFW.COM) – Historians and political experts are reacting to an announcement by President Donald Trump that breaks a long-standing tradition.

When Joe Biden takes the oath of office on Jan. 20, President Trump says he won’t be there.

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The president announced the news Friday, Jan 8 on Twitter.

He said, “To all of those who have asked, I will not be going to the Inauguration on January 20th.”

When Trump was inaugurated in 2017, then President Barack Obama was there out of respect for a tradition that only three presidents throughout history have scorned, the most recent was Andrew Johnson when Ulysses S. Grant assumed the office in 1869.

Historians say it’s an important symbol of civility in the American political system.

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Jeff Engel, Director of the Center for Presidential History at SMU, said, “If you told me that President X, I did not know their identity, would not show up for their successors inauguration five years ago, I would’ve said, ‘that’s crazy, of course they would show up.”

He says previous presidents, even those who personally disliked each other like Herbert Hoover and Franklin Roosevelt, understood the important of appearing together.

“They didn’t talk that entire day but they stood together and they sat together and they had pictures taken together,” he said. “The image is what matters.”

Some former president’s, like George H.W. Bush have even left heartfelt letters of support to those they lost elections to.

As for President-elect Joe Biden, he says he doesn’t care personally whether Trump attends but thinks it would be better for the nation if he did. 

He recently discussed the matter with CBS Late Show host Stephen Colbert. 

“We’ve been the beacon of democracy for the last 200 years and the peaceful transfer of power and the power being handed from one person to another, one party to another is who we are,” said Biden.” And that’s the part I worry about is how that reflects around the world and so, but that’s the only standpoint from which I don’t take it personally, but I do think it affects the view of the rest of the world as to who we are.”

“We are, even as competitors, Americans,” said Engel. “For the past century-plus, we have had the tradition that people who compete against each other for election are capable of maturely shaking hands.”

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