President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE on Monday accused former Vice President Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE of lying about his lack of a 2020 endorsement from former President Obama in an exclusive on-camera interview with Hill.TV.

"How he doesn’t get President Obama to endorse him — there has to be some reason why he’s not endorsing him," Trump said outside the Oval Office. "He was the vice president. They seem to have gotten along. And how President Obama's not endorsing him is rather a big secret," Trump mused, adding, "Then he goes and lies and said, 'I asked the president not to endorse me.' Give me a break."

Trump argued Biden made the assertion because "he’s embarrassed by the fact that Obama’s not endorsing him, so he goes out and says, 'I asked President Obama not to endorse me.' ... So it could be that President Obama knows something, but there is something going on in that brain of his."

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Pressed on what exactly is going on in Biden’s brain, Trump said, "I think he’s off. He’s different. We’ve all known him a long time. I’ve seen him for a long time. Frankly, he looks different, he sounds different, and he thinks different. Other than that, I hope he does very well."

Trump in a recent interview with "Fox & Friends" declined to endorse his own vice president, Mike Pence Michael (Mike) Richard PencePence vows for law and order everywhere Trump met with chants of protest as he pays respects to Ruth Bader Ginsburg The Hill's 12:30 Report: Ginsburg lies in repose MORE, ahead of a possible run for president in 2024. Asked about his comments by NBC News's Chuck Todd Charles (Chuck) David ToddMurkowski: Supreme Court nominee should not be taken up before election Republican senator says plans to confirm justice before election 'completely consistent with the precedent' Sunday shows - Trump team defends coronavirus response MORE, Trump said he didn't answer because it was a "surprise question."

"I mean it's so far out. That would be the only reason. Now what happens in 2024 I don't know that Mike is going to run. I don't know who's running or anything else," the president said.