WASHINGTON, United States (AFP) — Kamala Harris celebrates her 60th birthday on Sunday, but it’s US election rival Donald Trump whose age is under scrutiny after a series of rambling speeches and a bizarre dance party.

Seeking to shift the polls in a desperately tight White House race, Harris has ramped up attacks on the mental fitness of the 78-year-old, who is the oldest presidential candidate in US history.

“Donald Trump is increasingly unstable,” Democratic vice president Harris said on Wednesday after Trump spent nearly 40 minutes on stage swaying to music at a town hall event.

Harris is unlikely to take time off the campaign trail for her birthday as she and Republican former president Trump hammer crucial battleground states that could decide the November 5 vote.

US presidential elections are a major test of stamina for any candidate — but the 2024 race has been unusually focused on age.

Harris’s remarkable ascent to the top of the Democratic ticket came after 81-year-old President Joe Biden was forced to throw in the towel after a disastrous debate performance against Trump.

Biden’s replacement as nominee by a vice president two decades his junior provided a shot in the arm for the party’s election hopes, as Harris chased down Trump in the polls.

Trump said as much shortly after Biden pulled out.

“She is younger,” Trump told Fox News at the time. “I mean, she’s 60 years old… I didn’t realize she was 60. I thought she was a little younger. But she is 60.”

But with the polls stagnant for weeks, Harris has played the age card more and more.

The Democrat released her own medical report on October 12, describing her as in “excellent health” and fit for the presidency, with the only minor health issue of note being seasonal allergies and hives.

She then repeatedly pushed Trump over his own refusal to do the same.

– ‘Sexism against older women’ –
The billionaire has instead referred to a note issued by his former White House doctor last year and his campaign insists on his stamina.

Harris’s campaign has increasingly insinuated that Trump is not up to a second term, citing his withholding of his medical history plus a series of canceled television interviews and political events.

“What is Donald Trump trying to hide?” Harris said on X.

Her campaign has also highlighted Trump’s increasingly dark and erratic stump speeches and interviews, including when he recently raised the possibility of using the US military against the “enemy from within.”

Polls suggest it’s a strong issue for Harris.

Pew Research Center polling showed the percentage of people who call Trump “mentally sharp” dropping from 58 percent in July to 52 percent in September.

And while 49 percent of voters said Trump’s age would hurt his candidacy, 46 percent of voters said Harris’s age would help hers.

But the balancing act is also a tough one in an election where the age of voters is also a key issue.

Harris commands solid support among younger voters but older voters who may find her attacks on Trump’s age lean towards him — in a recent CBS poll, voters aged 45-64 side with Trump over Harris 53 percent to 46, while among voters ages 65 and over it grows to 57 percent to 42 percent.

Yet Harris too faces underlying prejudices about age, said Nancy Hirschmann, a professor of American social thought at the University of Pennsylvania.

“Her age does have an effect in one sense, in that, the sexism of America really is for sure on older women,” Hirschmann told AFP.

Trump’s running mate J.D. Vance found himself under scrutiny for comments from a few years back in which he called top Democrats “childless cat ladies” and appeared to agree with dismissive remarks about post-menopausal women.

Harris has two stepchildren from her marriage to “Second Gentleman” Doug Emhoff.

“There’s an added layer of sexism against older women over 50,” added Hirschmann.