The decision leaves Mitt Romney as the incontestable nominee to face Barack Obama in next November’s polls.

US Republican presidential candidate, Rick Santorum on Tuesday April 10, 2012 ended his bid for the White House, leaving Mitt Romney as the presumptive nominee, the BBC reported. The former Pennsylvania senator made the announcement at a news conference in the city of Gettysburg.

According to The Sun newspaper of Britain, Santorum told the press that his daughter's illness had made him and his family to have a rethink about the race, noting that he had won 11 states against all odds. His three-year-old daughter, Bella was hospitalised over the long holiday weekend with Trisomy 18, a rare genetic condition that hinders a child's development.

The Independent newspaper said though the timing of the announcement came as a surprise, it was already clear that he stood no chance against Mitt Romney who is better organised and financed. A staunch social conservative, Santorum was considered an underdog until he made his way to near the top of the Republican race and won the first 2012 nominating contest in Iowa by a thread. His rise forced the issues of birth control and the role of Christianity in public life to the forefront of the campaign, frustrating Romney who sought to keep the focus on the economy, Reuters news agency said.

Santorum’s vocal opposition to gay marriage and abortion offered Republican voters a stark contrast to Romney's more moderate record. When he criticised gay rights and birth control, the media highlighted a 2008 speech in which he declared that Satan was attacking great US institutions by means of pride, vanity, and sensuality.

Santorum lagged behind Romney in opinion polls and in the fight for the 1,144 party delegates needed to win the Republican nomination. He was facing the possibility of an embarrassing defeat in his home state of Pennsylvania on April 24. Romney has 659 delegates to 275 for Santorum, according to a CNN estimate.

Observers say to all intents and purposes, the Republican race is now over. With Rick Santorum out, Mitt Romney can focus solely on Barack Obama. His message is that the president has failed America on the economy - burdening small businesses with tax and regulations, while widening the deficit through what he says is an obsession with government spending.