Profile: Rick Santorum
Rick Santorum, the most enduring challenger to the Republican presidential candidacy of Mitt Romney, has suspended his bid for the White House.
After a series of recent losses in key state primaries, Mr Santorum had faced a widening gap with Mr Romney in the all-important delegate tally.
Mr Santorum had accumulated 285 delegates compared to Mr Romney's 661 when he announced his decision to withdraw from the race.
Rick Santorum made his mark on the Republican presidential race when he won an improbable victory in Iowa by just 34 votes.
The 53-year-old former Pennsylvania senator was written off just a few weeks earlier, before a surge in the polls drove him ahead of Mitt Romney. The margin was so close it was not certified until more than two weeks after the caucuses.
Despite a lull in the rest of January's votes, Mr Santorum swept back into the limelight with a triple win in Colorado, Minnesota and Missouri.
He followed that with victories in Oklahoma and Tennessee, but lost crucial states like Michigan and Ohio, and trails far behind in the delegate count.
His triple loss in the Wisconsin, Maryland and Washington DC primaries dealt the Santorum campaign a further blow.
'Consistent and persistent'
Mr Santorum represents the socially conservative wing of the Republican Party.
This is a group that has demonstrated its dissatisfaction with the moderate front-runner Mr Romney by shuffling its allegiances among so-called "Not Romney" candidates continuously for months.
But while the former Massachusetts governor has a multi-million-dollar campaign machine, Mr Santorum's organisation was run on a comparative shoestring.
Born in Virginia to an Italian father and a mother of Irish-Italian descent, Mr Santorum was raised in the blue-collar town of Butler, Pennsylvania.
His two Catholic parents worked at the local Veterans' Administration Hospital, where his mother was a nurse.
His brother once said that one of the key childhood lessons their father, a psychologist, gave them was to think positive.
He said that as a child, Mr Santorum had a deeply competitive streak which was evident when he played baseball, chess and board games.
After graduating from Penn State University with a degree in political science, he earned a master's degree in business administration in Pittsburgh and subsequently a law degree.
Terry Madonna, a professor of politics for 30 years in Pennsylvania, first met Mr Santorum in the 1980s when he worked for a Republican state senator.
"He was strong-willed, he knew what he was and what he wanted to accomplish. Sometimes what he said and did bordered on arrogance but he has strong principles.
"You may not agree with him but he's consistent and persistent. He's not a hypocrite."
Mr Santorum was elected to the House of Representatives in 1990 at the age of just 32, and four years later took Pennsylvania's Senate seat.
He held it for 12 years, making his name as a staunchly conservative politician. Along the way he has provoked criticism for his strong opposition to gay rights.
In 2005, Time magazine named the father of seven as one of America's 25 most influential evangelicals.
A year later he lost his seat in Congress by a huge margin, but he said his defeat was part of the national backlash post-Iraq and not a reflection on his own service.
Since then, he has been practising law and preparing for his presidential run.
The pre-Iowa Republican race did not give Mr Santorum much of the spotlight as a succession of others vied to be torch-bearer for the social conservatives.
That changed after his caucus wins and with a narrower field, he faced the kind of attacks from the Romney camp that comes with being at the front of the pack.
He was not able to apply the same focus to New Hampshire and South Carolina that he did to Iowa, where he campaigned hard, speaking at more than 350 campaign events.
Nevertheless, his focus on caucus states and appeal in the socially conservative Mid-West paid some handsome dividends.
Mr Santorum even gained a fleeting endorsement from Rupert Murdoch.
Prior to the vote in Iowa, the influential media mogul and owner of Fox News used his new Twitter feed to signal his approval: "Can't resist this tweet, but all Iowans think about Rick Santorum. Only candidate with genuine big vision for country."