WASHINGTON — White House press secretary Karine Jean-Pierre insisted Wednesday that President Biden remains sharp as a tack and “absolutely” could serve another four-year term — stunning journalists at her first briefing since Biden dropped his re-election campaign over concerns about his mental acuity.

“Look, he didn’t step down from campaigning or from running because he didn’t believe he can serve in a second term. That is not why,” Jean-Pierre claimed, to the incredulity of the press corps.

The press secretary said the 81-year-old president’s decision to drop out Sunday and endorse Vice President Kamala Harris, 59, was in the “best interest” of the country and the Democratic Party and that viewers should “stay tuned” for a fuller explanation from Biden in an 8 p.m. Oval Office address on Wednesday.

“So today, the president believes that he is capable, fully capable of serving in the office that he currently has now and for the next 4.5 years if he wanted to?” an Associated Press journalist asked Jean-Pierre.

Follow The Post’s live blog for President Biden’s first speech since dropping out of the 2024 race

“Absolutely,” she said.

Moments later, another reporter pressed, “Why is it in the country’s best interest for him to step aside? Does it have to do with his health?”

“It has nothing to do with his health,” Jean-Pierre claimed.

“The president’s going to speak to this directly to the American people tonight in prime time.”

Asked about allegations that Biden’s inner circle had covered up his cognitive condition, she asserted, “it is not a cover-up” and that it was “ridiculous” for public figures to suggest he resign before his term ends in January.

Jean-Pierre’s claims about Biden’s fitness came despite the president dropping out specifically in response to concerns about his mental acuity following a dismal June 27 debate performance in which he appeared confused and made remarks such as that he “finally beat Medicare.”

What to know about President Biden's decision to drop out of the 2024 presidential race:

• President Biden announced Sunday he is dropping out of the 2024 presidential race — after weeks of prominent Democrats and donors calling on him to withdraw following his disastrous performance in the first presidential debate.
• In a letter posted on X Sunday afternoon, Biden admitted that it is in the “best interest of my party and country” for him to step down as the Democratic nominee.
• Biden wrote that he intends to serve out the remainder of his term and will address the country on his decision later this week.
• In a follow-up X post, Biden endorsed Vice President Kamala Harris for the suddenly open Democratic nomination. “Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year,” the president wrote.
• Former President Donald Trump reacted to the news by labeling Biden the “worst president in the history of our country” in a call with CNN.

Biden compounded questions about his fitness on July 11 when he introduced Ukrainian President Volodymyr Zelensky to NATO leaders as “President Putin” of Russia — and then referred to Harris as “Vice President Trump” at a press conference.

Leading Democrats such as former House Speaker Nancy Pelosi (D-Calif.), Senate Majority Leader Chuck Schumer (D-NY) and former President Barack Obama made clear their preference that Biden stand down to avoid a possible landslide defeat, dragging down other down-ticket Democrats.

Even before Biden’s calamitous debate, most voters were concerned about the president’s age.

An overwhelming 86% of respondents said Biden is too old for another term in a February ABC/Ipsos poll and 73% said the same in a March Siena College/New York Times survey.