Trump insists he isn't mixing up Obama and Biden as critics try to spotlight his gaffes

Former President Donald Trump has made calling out rival Joe Biden's age and mental fitness a big part of his comeback campaign for the White House. But on Monday, he went on the defensive himself.

In a social media post, Trump wrote that despite what critics claim, he hasn't repeatedly mixed up the names of former President Barack Obama and current President Biden in his campaign speeches and other appearances.

On the trail over the last few months, Trump has said "Obama" multiple times when he appears to be referring to Biden, drawing attention from his detractors, including Trump's Republican rivals and Biden's aides, who are increasingly trying to spotlight Trump's own gaffes as age continues to be a central issue in the 2024 election.

Most recently at an event on Veterans Day in Claremont, New Hampshire, Trump called Obama the "current president" while talking about former Fox News host Tucker Carlson's interview with Hungarian leader Viktor Orban.

Last month in Iowa, Trump said "Obama administration" while touting his role in building walls along the southern border; and in September, at a campaign event in Washington, D.C., Trump appeared to say that he was leading Obama in the polls.

In a social media post on Monday, Trump said that he "sarcastically" replaces Obama's name for Biden's when talking about the president of the United States -- not because he doesn't know their names.

"I know both names very well, never mix them up, and know that they are destroying our Country," Trump wrote.

In his post, he also floated the baseless conspiracy that Obama, who left office six years ago, retains influence in the Biden administration.

Responding to Trump's social media post, a Biden adviser pointed back to other Trump gaffes. "Is that also his explanation for not knowing World War II happened? Or that Turkey's a different country than Hungary?" the adviser said, in part. They also referred to when Trump infamously suggested that people could inject disinfectant in order to treat COVID-19 in the early days of the pandemic.

"Remember not to inject bleach, kids," the adviser said.

Trump's explanation echoes his recurring campaign rhetoric over the past few months -- which ties into his attacks on Biden's mental and physical health.

"Who's his boss? Barack Hussein Obama, Barack Hussein, Barack Hussein Obama," Trump said during a campaign rally in Florida in October.

Notably, during a Fox News Radio interview last month, interviewer Brian Kilmeade corrected Trump when he mentioned Obama while seeming to mean Biden.

"Well, you mean President Biden," Kilmeade said, to which Trump retorted: "Obama's Biden's boss."

There has been no evidence that Obama has any presence in directing White House decision-making, though he has assisted in the administration on topics he worked on while serving as president, most recently helping draft a new White House policy on artificial intelligence.

Polling shows Americans have concerns about the advanced age of both Biden and Trump, who are currently the front-runners to be their party's nominees in next year's election. However, more people have issues with the age of Biden, who is the oldest-ever president and would be 82 at the start of a second term.

In an ABC News/Washington Post poll conducted in September, 74% of Americans said Biden was too old for reelection -- up 6% since May -- while Trump was seen as too old by 50% of Americans.

A Trump campaign spokesperson previously said in a statement that his gaffes are different from the ones that he and his allies highlight of Biden, saying "false narratives" haven't affected Trump's standing.

"People know President Trump is the strongest candidate," spokesperson Steven Cheung said in October, in part.

Noting some of Biden's past snafus, such as tripping on Air Force One's staircase, Cheung continued then: "There's no correcting that and that will be seared into voter's minds."

Biden has said it's fair for people to consider his age, but he's also defended his stamina and record.

"I feel good. I feel excited about the prospects, and I think we're on the verge of really turning the corner in a way we haven't in a long time," Biden told ABC News Chief White House Correspondent Mary Bruce in April.

ABC News' Fritz Farrow contributed to this report.