The "Partridge Family" star was admitted to a Florida hospital three days ago and is suffering from multiple organ failure.
The 67-year-old actor, singer and former 70s teen idol is "currently conscious and surrounded by family", his rep told E!
"He is hoping and waiting for a liver transplant, if he happens to get one in time, the prognosis is still up in the air", the source told Us.
Cassidy announced early this year he had been diagnosed with dementia.
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David's family have apparently been arriving at the hospital to comfort him.
He went on to become a successful singer garnering a loyal fan base and performing many of his hit songs from "The Partridge Family", such as "I Think I Love You".
Cassidy kept touring and recording after his role as Keith Partridge ended in 1974, performing on Broadway in NY, in London's West End and in Las Vegas. Both his liver and kidney are failing, according to his rep.
"I was in denial", Cassidy said, "but a part of me always knew this was coming".