Santorum made the announcement after stepping away from the campaign this weekend to be with his 3-year-old daughter, Bella. She was released from a Virginia hospital and is settling in at home with her parents, a campaign spokesman said Tuesday.
Santorum did not campaign Monday so he could be with his daughter, who was discharged from the hospital Monday night.
Gidley said Santorum and his wife, Karen, were "truly overwhelmed by the prayers and support they've received."
In ending his bid for the GOP 2012 nomination Santorum described his daughter as "a fighter and doing exceptionally well." But he added her illness "did cause us to think about our role as parents. This was a time for prayer and thought this past weekend. Just as it was when we decided to get in this race,"he said.
Santorum was facing an uphill battle against front-runner Mitt Romney in the race for the Republican presidential nomination. Five states, including Santorum's home state of Pennsylvania, hold primaries April 24.
Romney had planned to spend $2.9 million in TV ads in Pennsylvania. But in deference to Bella's illness, Romney's campaign pulled down a harsh ad that was running against the former Pennsylvania senator there. Romney spokeswoman Andrea Saul said Monday the campaign asked TV stations over the weekend to pull the ad and replace it with a positive, pro-Romney spot.
Romney was far ahead of Santorum in the race for delegates to the Republican National Convention and is now the party's likely nominee.