Of all the issues that seemingly preoccupy the mind of Donald J. Trump — his perceived persecutions, the evil of his enemies, the size of the rallies — there is one that may be the most consistent: other people’s looks.

The former president’s obsession with personal appearance dates back decades, but has flared anew with the candidacy of Kamala Harris, a Democrat whose looks he has repeatedly mentioned this month, comparing his opponent’s appearance on the cover of Time magazine to that of “the most beautiful actress ever to live.”

Mr. Trump also compared her face on the Time cover to Sophia Loren, his wife Melania, and then, finally, in a jaw-dropper for a man known for such jaw-droppers, asserting that he was “much better looking than her.”

“Much better,” Mr. Trump, 78. “I’m a better-looking person than Kamala.”

Following this logic, that also makes him better than his former-model wife, in his estimation. And while such a statement was met with astonishment and amusement from his Democratic opponents, Mr. Trump’s fixation on other people’s looks has been used as both a devastating political tool and — specialists in male psychology say — a means of bolstering his own sense of self.

“I don’t think there’s ever been a president who was more obsessed with other people’s appearance,” said Michael Kimmel, a professor emeritus at Stony Brook University and the author of “Manhood in America.” “This is a man for whom appearances are everything, from his fake tan to his haircut to lying about his weight. He’s obsessed with physicality.”

Others have remarked on Ms. Harris’s appearance. At the Democratic National Convention this week, Bill Clinton described Ms. Harris as having a “thousand watt smile.” And even back in 2013, Barack Obama made news when he said she was “by far the best looking attorney general in the country.”