'This is not a sane man': Trump brutally mocked for 'projection' after questioning Biden's mental fitness

President Donald Trump slurred Democratic frontrunner Joe Biden as "old" and "slow" -- and social media users noticed he was projecting his own apparent weaknesses on his potential opponent.

Trump questioned Biden's fitness for office during a press scrum outside the White House, before heading to Iowa, where he and the former vice president will hold dueling campaign rallies.

"I think he's the weakest mentally," Trump said, pointing to his own head. "I like running against people that are weak mentally. I think Joe is the weakest up here."

"He’s a different guy," Trump added. "He acts different than he used to. He looks different than he used to. I think Joe is weak mentally."

Twitter users immediately pointed out those were Trump's own weaknesses.