Julian Zelizer is a history and public affairs professor at Princeton University and co-host of the "Politics & Polls" podcast. In January, Norton will publish a new book by him and Kevin Kruse, "Fault Lines: A History of the United States Since 1974." Follow him on Twitter: @julianzelizer. The opinions expressed in this commentary are those of the author. View more opinion articles on CNN.

(CNN) President Donald Trump forfeited the moral authority of the presidency from his first day in office. The issue has not been that Trump doesn't know about this important presidential role -- for our commander in chief to stand as a guide during times of crisis -- but rather it has never been of interest to him. For Trump, the presidency is a raw tool he has used to achieve political power, and the office is a partisan bludgeon to keep the other side on the defense. The presidency is about power for its own sake.

Julian Zelizer

This aspect of the Trump presidency is not as consequential on many days as it is on others. But during those moments when the nation does turn to the President for moral guidance and support, it is triply stunning to observe a leader who cares so little about filling this role.

Most presidents who have been in office during these kinds of traumatic moments have lived up to the challenge. President Ronald Reagan brought many people in the nation to tears with a speech after the explosion of the space shuttle Challenger in 1986 that killed everyone on board.

President Bill Clinton had poignant words following the terrorist attack on the federal government building in Oklahoma, while President George W. Bush stirred a grieving country when he stood at Ground Zero in 2001 with first responders and promised them he would take action.

When President Barack Obama sang "Amazing Grace" at the 2015 memorial service for the African-American churchgoers who were brutally killed by Dylann Roof, he used the power of his office to bring some feeling of healing and calm.

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