The announcement comes the day after Santorum's 3-year-old daughter Bella was released from the hospital -- her second trip this year. Bella suffers from a rare and often fatal disorder called Trisomy 18.
"This was a time for prayer and thought over this past wekend," Santorum said. "Just like it was when we decided to get into this race ... we were very concerned about our role of being the best parents we possibly could to our kids," Santorum said of the kitchen-table discussion with his family before he launched his campaign.
Santorum said he balanced his desire to be a good father with his desire to do something positive for the country. He mentioned Bella and her condition as one of the reasons he joined the race -- to look out for Americans like her, who he said are "left behind."
"While this presidential campaign is over for me, we are going to continue fighting for those voices," Santorum told a sedate crowd in Gettysburg, Pa.
Santorum waged a scrappy and unexpectedly strong campaign for the Republican nomination, essentially moving to Iowa before narrowly winning the caucus there. Barely registering on opinion polls throughout 2007, he was the first candidate to visit all of the state's 99 counties. He built his campaign around engaging audiences in townhall meetings, often wearing a sweater vest and uttering his campaign battle cry, "Game on."
But like many of his stronger showings in the primaries, Santorum' s Iowa victory was marred by some bad luck; party officials there initially called Mitt Romney the narrow victor on caucus night, only to later give the nod to Santorum.
In total, Santorum has won ten presidential preference contests.That's one fewer than Romney carried in his losing bid in 2008. Santorum has recently compared his current run to Ronald Reagan's in 1976. Reagan carried his attempt to unseat President Gerald Ford all the way to the Republican convention that year. Ford eventually got the nomination, but lost against Democrat Jimmy Carter. Reagan was the Republican nominee four years later in 1980.
Party elders had begun to coalesce around Romney and were urging Santorum to end his campaign in recent days. Everyone from forrmer President George H.W. Bush to Florida Sen. Marco Rubio and Tea Party Leader Sen. Jim DeMint had urged Republicans to view Romney as the party's choice.
Today, RNC Chairman Reince Priebus praised Santorum for ending his campaign.
"Today, Senator Santorum has made a commendable decision. He has decided to put his country, party, and desire to defeat President Obama ahead of any personal ambition. I applaud his decision and congratulate him on the campaign he has run," said Priebus in a statement.
Santorum went on to become the main conservative alternative to Romney. But Romney and his allies outgunned Santorum in television advertising and outmaneuvered him in the delegate race.
Santorum's move makes Romney, the former Massachusetts governor who has a commanding delegate lead, the all but certain Republican nominee, although Newt Gingrich and Ron Paul both remain actively in the race.