"Rick and his wife Karen have taken their daughter Bella to the hospital," Santorum spokesman Hogan Gidley said in a written statement. "The family requests prayers and privacy as Bella works her way to recovery."
Three-year-old Bella suffers from Trisomy 18, a rare genetic disorder that is fatal for about 90 percent of children before or during birth.
The toddler was admitted to the hospital in January with pneumonia and Santorum briefly suspended campaigning.
It's unclear why the child was hospitalized Friday, ABC News said.
Santorum often discusses his youngest child while campaigning, noting that she had multiple hospital visits before her first birthday.
Santorum had taken a break from campaigning for the Easter weekend. Before his daughter's hospitalization was announced, Santorum posted on his Twitter page he would resume campaigning Monday.