 In a letter posted on X, the president said it was time for him to step down and focus on the rest of his presidency. Biden also announced he's endorsing his vice president Kamala Harris, but that does not mean she will be the official Democratic nominee. Democratic leaders for weeks have expressed concerns about the 81-year-old's mental fitness and his path to victory over Donald Trump. Biden was diagnosed with COVID on Wednesday and has been in self-isolation at his beach house in Delaware.