Eight months in, it’s hard to argue that the presidency has changed Donald Trump.

The bombastic former reality television star hasn’t been sobered by daily intelligence briefings, or by North Korean nukes. In Houston, confronted with human suffering on an unimaginable scale, he mostly talked about himself. He continues to boast about his crowd sizes, his gilded Manhattan apartment and his golf courses. Other presidents were awed by the history of the house they inhabited—Trump called it “a real dump.”

On the eve of the election, Barack Obama warned voters that the presidency doesn’t change a person. “Who you are, what you are, it doesn’t change after you occupy the Oval Office,” he said. “It magnifies who you are. It shines a spotlight on who you are.” 

But a virtually forgotten American president was an exception to Obama’s rule. Like Trump, he was a wealthy New Yorker, disparaged by big-city intellectuals as unqualified, unfit and corrupt. Fellow Republicans were shocked when he landed on the threshold of the highest office in the land—but no more shocked than he was. 

The majority of Americans viewed his ascension with dread, and leading newspapers feared for the future of the republic. The Chicago Tribune lamented “a pending calamity of the utmost magnitude.” The New York Times called him “about the last man who would be considered eligible” for the presidency. Newspapers in Charleston and Louisville said he was a criminal who belonged in jail—or worse.

Chester Alan Arthur, the nation’s 21st president, surprised them all.  ...