It’s Donald Trump’s 78th Birthday. He Isn’t Happy About It.

“Donald hates his birthdays,” Ivana Trump once said of her ex-husband.

He seemed to hate this one, on Friday, especially. He looked genuinely upset on Sunday when his supporters sang an early “Happy Birthday” to him at a rally in Las Vegas. “You know,” he told them, “there’s a certain point at which you don’t want to hear ‘Happy Birthday.’ You just want to pretend the day doesn’t exist.”

No doubt. Now 78, he is trying to convince the country to give him another four years in office, at the end of which he would be the oldest president in American history — a mantle held by the current, 81-year-old occupant of the Oval Office.

On the campaign trail, both men try to downplay their age while making the other appear as creaky and cranky as possible.

The Trump team spent the week blasting out selectively edited videos of Mr. Biden toddling around Europe, looking like Corrado Soprano lost in Newark. “Biden was seen staring off into the distance and wandering around like a brain-dead zombie,” said Karoline Leavitt, a Trump campaign press secretary.

The Biden campaign hit back with an email on Mr. Trump’s birthday reminding voters that he fell asleep at his own trial, a case that resulted in felony convictions. “He is unhinged, unable to focus and diminishing before our very eyes,” James Singer, a Biden campaign spokesman, said of the former president.

Mr. Trump used to have fun on his birthdays. “Donald was the child who would throw the cake at the birthday parties,” his younger brother Robert once told the journalist Marie Brenner. After he became a mogul, Mr. Trump would throw swinging parties at his casinos in Atlantic City, like the one at Trump’s Castle in 1988, for his 42nd birthday, when he played videos of Liza Minnelli and Billy Crystal wishing him a happy birthday. (President Ronald Reagan sent a telegram.)

Two years later, Mr. Trump’s birthday party was hosted by Robin Leach, from the TV show “Lifestyles of the Rich and Famous.” Dolly Parton and Elton John sent videos, Andrew Dice Clay performed, and a George H.W. Bush imitator declared that Mr. Trump should one day be president. At Mr. Trump’s 50th birthday celebration, in Trump Tower in New York, Eartha Kitt sang to him, and there was an ice sculpture of Marla Maples, then his wife, in mermaid form.

There are not likely to be any mermaids or Catwomen at his 78th. It’s being held at the Palm Beach County Convention Center near the airport by a group of supporters who call themselves Club 47 USA, a hopeful reference to Trump’s aspiration to become the 47th president. General admission cost $25.

These are the people with whom Mr. Trump seems most at ease these days. The last time he addressed this group, in October, he opened up about the day he ordered a drone strike on Iran’s top security and intelligence commander, Maj. Gen. Qassim Suleimani.

“Nobody’s heard this story before,” Mr. Trump told them. “But I’d like to tell it to Club 47, because you’ve been so loyal.” He also talked at the event about his disappointment with Prime Minister Benjamin Netanyahu of Israel — even though it was only four days after the Hamas-led Oct. 7 attack on the country. “I’ll never forget that Bibi Netanyahu let us down,” Mr. Trump said then.

On Thursday, Mr. Trump was celebrated by a group that has not always been as loyal as Club 47: Republican lawmakers. At a meeting on Capitol Hill, House Republicans serenaded him, and senators presented a vanilla-frosted cake with big “45” and “47” candles stuck in it. But there is something about turning 78 that makes a man contemplate his mortality.

At the rally in Las Vegas, Mr. Trump had, in a rare moment of introspection, riffed about his old age.

“My father lived a long time, my mother lived a long time, and they were happy and they were great,” he said, sounding a bit muted. “So maybe we’re going to live a long time. I hope so.”