Trump, 78, just called himself a 'fine and brilliant young man'

• With Joe Biden out of the race, Donald Trump has become the oldest presidential nominee ever.
• Trump's now facing off against Kamala Harris, 59, a woman decades younger.

Former President Donald Trump, 78, is the oldest presidential candidate ever now that his former opponent, President Joe Biden, is out of the race.

But the businessman-turned-politician is going all out to prove that age is just a number — particularly as he now contends with the idea of facing off against Vice President Kamala Harris, 59.

On Monday, Trump took to Truth Social to excoriate Harris, who is currently primed to replace Biden as Democratic nominee.

"Lyin' Kamala Harris, the Biden appointed 'Border Czar' who never visited the Border, and whose incompetence gave us the WORST and MOST DANGEROUS Border anywhere in the World, has absolutely terrible pole numbers against a fine and brilliant young man named DONALD J. TRUMP!" Trump wrote.

When approached for comment, a Trump campaign spokesperson pointed Business Insider to a set of polls that showed Trump leading Harris in Michigan, Pennsylvania, Arizona, and North Carolina.

While there may be room for debate on the Biden administration's handling of the southern border crisis, it's notable that Trump is trying to advertise himself as "brilliant" and "young."

At 59, Harris is nearly 20 years younger than the GOP nominee.

This isn't the first time Trump has tried to use his own age as a selling point, though the attack made a lot more sense when he was up against older Democratic contenders such as Biden or Sen. Bernie Sanders of Vermont, 82.

"I'm so young. I can't believe it, I'm the youngest person," Trump told reporters in April 2019, ahead of the 2020 presidential elections. "I am a young, vibrant man. I look at Joe, I don't know about him."

To be sure, Trump's scathing reaction to Harris could stem from a concern that she may be a harder candidate to beat than his campaign's preferred choice, Biden.

For one, Trump's campaign advisor Susie Wiles called Biden a "gift" in an interview with The Atlantic's Tim Alberta in mid-June.

And, in a subsequent story published on Monday, Alberta said Wiles once told him she was more afraid of the other talents in the Democratic Party than Biden.

"I don't think Joe Biden has a ton of advantages. But I do think Democrats do," Wiles told Alberta on Super Tuesday this year, which took place on March 5.

Representatives for Harris didn't immediately respond to a request for comment from BI sent outside regular business hours.