The president was transported to the Walter Reed Medical Centre after being collected from the White House lawn by Marine Force One. Gabriel Sherman, a special correspondent for Vanity Fair, said the President feared for his life during this time.
He tweeted: “Conservations with Republicans close to White House over last 12 hours indicate it’s been far more dire than White House has said.
“Before being taken to Walter Reed Trump kept asking aides: ‘Am I going out like Stan Chera? Am I?’”
Stanley Chera, a New York property developer and friend of Mr Trump, died on April 11 from coronavirus related complications.
The president first confirmed he and the first lady had tested positive for coronavirus on Thursday.
A number of key Trump allies, including campaign manager Bill Stepien and former counsellor to the President Kellyanne Conway, also have the disease.
On Saturday Mr Trump revealed he is feeling “much better now” in a video recorded at the Walter Reed Medical Centre.
In the 3.49 minute clip published on Twitter the president said: “I came here, wasn’t feeling so well. I feel much better now.
“We’re working hard to get me all the way back. I have to be back because we still have to make America great again.
He added: “At this time the team and I are extremely happy with the progress the president has made.”
However White House chief of staff Mark Meadows said the president’s condition over the past 24 hours had been “very concerning” and the next couple of days will be critical.
Mr Biden is competing against Democratic candidate Joe Biden in November to retain the presidency.
In a statement Mr Biden said: “Jill and I send our thoughts to President Trump and First Lady Melania Trump for a swift recovery.
“We will continue to pray for the health and safety of the president and his family.
“I’m happy to report that Jill and I have tested negative for COVID.
“Thank you to everyone for your messages of concern.
“I hope this serves as a reminder: wear a mask, keep social distance, and wash your hands.”
Mr Biden was tested after debating with Mr Trump on Tuesday in Cleveland.
Critics of the president argue he downplayed the threat from coronavirus early on in the pandemic.
In later February Mr Trump commented: “It’s going to disappear. One day — it’s like a miracle — it will disappear.”