Cassidy reportedly checked into a Florida hospital three days ago, and his family has been told to "prepare for the worst". Cassidy's character sang lead. I want to love.
Cassidy is part of a large showbiz family that included his father, Tony-winning actor Jack Cassidy, stepmother Shirley Jones, half-brother Shaun Cassidy and daughter, actress Katie Cassidy. Cassidy's own parents, Evelyn Ward and Jack Cassidy, were both actors.
After the show ended in 1974, he focused on recording and songwriting but continued to act in smaller roles.
The former teen heartthrob, now 67, is "currently conscious and surrounded by family", the rep notes.
Cassidy announced early this year he had been diagnosed with dementia.
Other media sites have reported Cassidy is suffering from organ failure.
His family are now said to be at the hospital to be by his side as his condition worsens. "That's when I began to be very concerned", Cassidy said on the program.