Rick Santorum, who pulled out of the race for the White House this morning, emerged as an improbable leading contender for the Republican nomination, with a faith-and-family message that caught fire with the party's most conservative voters.

Even as he quit the rollercoaster Republican presidential race, he remained the candidate who seemed to stir more emotions than most, with his radical views on religion, women and marriage.

His dark horse presidential bid which turned into a surprisingly strong challenge to frontrunner Mitt Romney, long considered the frontrunner but who faced a succession of challengers - most recently and perhaps most persistently Santorum - nipping at his heels.

"This presidential race is over for me,'' the former US senator told a crowd at a hastily convened press conference in Gettysburg, Pennsylvania.


Santorum now leaves a clear path to the nomination for the former Massachusetts governor, who is all but assured of locking up the Republican party's nomination to challenge President Barack Obama in November.

Despite his staid knitted sweater vest and easy boyish smile, Santorum was given to passionate outbursts when he talked about God and country which endeared him to right-wing conservatives.

But those same strong conservative views, born from his Catholic faith, are giving many pause as Republicans struggle to coalesce around one candidate to take on Democrat President Barack Obama in the November elections.

The 53-year-old former Pennsylvania senator was a virtual unknown when he first threw his hat into the ring in June last year to be the Republican Party's nominee.

Despite being written off early on, Santorum built his campaign state-by-state - a David and Goliath struggle against a Mitt Romney juggernaut powered by a huge war chest and a solid organizational machine.

But he faced the ignominious prospect of possible defeat in his home state of Pennsylvania, which is due to hold its presidential primary on April 24, after openly declaring on several occasions that it was a "must-win'' election contest for his foundering campaign. His exit on Tuesday, two weeks ahead of the vote, spares him that fate.

Santorum's pro-life, anti-contraception, marriage-only-between-a-man-and-a-woman message has gained traction with heartland evangelicals deeply sceptical of Romney, whom they view as a moderate disguised in conservative clothing.

Yet on the other end of the spectrum, critics saw his radical right-wing views as somewhat scary. A website called "Santorum exposed'' said it was dedicated to "shining a bright light'' on what it calls the former senator's "extreme positions.''

Santorum, a global warming skeptic, has called Obama "a snob'' because he believes all kids should have a college education, and said he wanted "to throw-up'' when watching former president John F. Kennedy talk about the separation of church and state.

"I don't believe in an America where the separation of church and state is absolute,'' he told US television recently.

At a campaign stop in Ohio last month, Santorum also hit out at Obama saying the president's "world view'' elevated the Earth above man.

"That's what I was talking about - energy: this idea that man is here to serve the Earth, as opposed to husband its resources and be good stewards of the Earth. And I think that is a phony ideal.''

Asked to define himself in one word at a recent debate, Santorum replied "courage.'' That steely resolve, refusing to stray off his message of "Family, Faith and Freedom,'' has defined his campaign even as it struggled.

A trained lawyer and father of seven who has been married to his wife Karen for 21 years, Santorum was first elected to the House of Representatives for Pennsylvania in 1990. He served two terms in the US Senate from 1995 to 2007.

In an intensely personal speech following his early win in Iowa, Santorum described his roots in Pennsylvania's coal country, where his grandfather worked in the mines until he died at 72.

"I knelt next to his coffin. And all I could do - eye level - was look at his hands. They were enormous hands. And all I could think was those hands dug freedom for me.''

Santorum has often spoken of his seriously ill youngest daughter, Isabella Maria, who suffers from a genetic disorder, trisomy 18 - or Edwards syndrome - which results in severe disabilities and abnormalities.

He had to break away from the campaign trial in January and again over the weekend to be by her bedside as the three-year-old recovered from treatment.

He has also tearfully recounted a family tragedy in 1996 when a son died hours after being born prematurely. He and his wife spent the night with the body and brought it home to show to their other children before burying it.

"Ask me what motivates me, it's been the dignity of every human life,'' Santorum said in his Iowa speech.