"Our daughter Bella's 4th bday is on Sun. Celebrate w/us! Tweet a bday wish w/#happybirthdaybella or send a wish," he wrote.
Bella, the youngest of Santorum's seven children, suffers from a chromosomal condition called Trisomy 18. Half of patients born with the condition do not survive past the first week of life, according to the National Institutes of Health.