Republican presidential candidate Nikki Haley warned her GOP rivals that they are actually challenging Vice President Harris for the Oval Office. On "Fox & Friends" Monday, Haley renewed calls for mental competency testing for older politicians and said Kamala Harris will be running the White House if Biden wins in 2024. 

BIDEN'S ‘WATCH ME’ MOMENT WILL HAUNT HIM IN 2024 AFTER RECENT FALL, REPUBLICAN STRATEGISTS SAY

NIKKI HALEY: I think everybody is trying to cover up for [Biden] because they think he's the only one that can beat Trump. That's ridiculous. I am telling you, we've got to get past having all of these people that are older in D.C. We've done that long enough. We need term limits. We need mental competency tests for people over the age of 75. I don't care if we do it for people over the age of 50, but these are people that need to be at the top of their game. They're making decisions on our national security. They're making decisions on our children's economic future. We can't continue down this path. And let's be very clear if they think it's going to be President Biden, a vote for President Biden, it's actually a vote for president Harris. We are running against Kamala Harris. Make no bones about it. The New York Times knows it. Every liberal knows it. They know that it's Kamala Harris that's going to end up being president of the United States if Joe Biden wins this election.

US Vice President Kamala Harris shows records she bought at Home Rule Records in Washington, DC, on May 3, 2023. (Photo by Mandel NGAN / AFP) (Photo by MANDEL NGAN/AFP via Getty Images) (Mandel Ngan)

Republican strategists are growing increasingly concerned over Biden's ability to serve the nation after he took a tumble last week at an Air Force event, less than a year after he told voters to "watch me" if they don't believe he has what it takes to fulfill his duties for another term in office.

Several Republicans have questioned whether the 80-year-old Biden – the oldest person to serve as president of the U.S. – is physically and mentally up to the intense demands of the White House for another four years. However, many of those questioning his abilities had done so well before he fell at the commencement ceremony at the U.S. Air Force Academy in Colorado last Thursday.

Last November, while fielding questions from reporters at the White House, Biden dismissed concerns from two-thirds of American voters who said in a poll that he should not seek re-election. "What is your message to them and how does that factor into your final decision about whether or not to run for re-election?" one reporter asked at the time. 

"It doesn't" Biden replied. Asked once more at the time what his message would be to those who oppose his endeavors to seek another term, Biden said, "Watch me."

Fox News' Kyle Morris contributed to this report.