Mr. Santorum returned to his home state to announce the end of his campaign.
Appearing before reporters in a small hotel conference room, the former U.S. senator from Pennsylvania said he and his family had arrived at the decision this weekend while he was home caring for his 3-year-old daughter Isabella, who had been hospitalized.
"We made a decision over the weekend that while this presidential race for us is over for me, and we will suspend our campaign effective today, we are not done fighting," he said.
He said his daughter, called Bella, is doing "exceptionally well," but that her illness led him to reflect about the race.
"This was a time for prayer and thought," he said.
Mr. Santorum spoke of the early days of his campaign in Iowa, when few political observers believed he would go on to win 11 states in the fight for the Republican nomination.
He did not mention Mr. Romney, although his strategist, John Brabender, told reporters that Mr. Santorum had spoken with the former Massachusetts governor.