Santorum pulls out

Mr. Santorum, a former senator from Pennsylvania, was trailing Mr. Romney in delegates and had little hope of catching up to him, but his strong performance in a brutal nominating contest established him as a force that the party will probably have to reckon with this presidential election year and beyond.

His departure from the race created an anticlimactic moment in the long presidential primary season for Mr. Romney, who has been actively seeking his party’s nomination for five years and found his conservative credentials constantly in question by the durability of Mr. Santorum’s candidacy. The move springs Mr. Romney from a political limbo in which he was acting like the nominee even though he faced the prospect of weeks of hard — and expensive — campaigning against Mr. Santorum. 

NY Times: Santorum Quits Race, Clearing a Path for Romney, April 10, 2012 

Santorum has been a stubborn opponent, winning a string of states, including Iowa, with his under-funded campaign contrasting with Romney's millions.

He won 11 states, including victories in Colorado and Minnesota in February and Alabama and Mississippi in March. His departure brings to an end a race that produced lots of ups and downs, with Republicans replacing one another as party favourite with on a frequent basis dating back to the Ames straw poll in Iowa in August. 

The Guardian: Rick Santorum bows to the inevitable and quits Republican presidential race, April 10, 2012

The Guardian: Newt Gingrich to Rick Santorum supporters: vote for me, April 10, 2012
The Guardian: Rick Santorum: campaign in quotes, April 10, 2012
NY Times: The Caucus Blog - Santorum Suspends Presidential Campaign, April 10, 2012