Republican presidential candidate Rick Santorum has ended his bid for the White House, leaving Mitt Romney as the presumptive nominee.

The former Pennsylvania senator made the announcement at a news conference in the city of Gettysburg.

"While this presidential race is over for me, we are not done fighting," said Mr Santorum, a social conservative.

He had been campaigning in Pennsylvania, his home state, ahead of its primary on 24 April.

But he was far behind Mr Romney in terms of funding and was in danger of losing the state for the second time in six years, analysts said.

In 2006 Mr Santorum lost his Pennsylvania Senate seat by an 18-point margin.

In the current race for the Republican nomination, Mr Santorum lags far behind Mr Romney in terms of the number of delegates needed to seal the nomination at the Republican convention in Tampa, Florida, in late August.

'Against all odds'

Mr Santorum's children and his wife Karen stood behind him in Gettysburg as he made the announcement that he was suspending his campaign.

Analysis To all intents and purposes, the Republican race is 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. The Obama campaign fully expected Mitt Romney to be the nominee. In a statement, it accused the multi-millionaire challenger of trying to buy the election - through negative adverts funded by special interests. And it claimed the more the American people saw of Mr Romney, the less they liked and trusted him. It all points to a brutal and very personal campaign.

He had taken time off the campaign trail in recent days as his three-year-old daughter Isabella, who has a rare genetic disorder, was admitted to hospital.

Mr Santorum proved to be the most resilient of the Republican rivals challenging Mr Romney's front-runner status.

In his statement he said he had surpassed expectations, adding that "against all odds, we won 11 states, millions of voters, millions of votes".

"We were winning in a very different way," Mr Santorum said, "We were touching hearts."

He remembered some of the volunteers he worked with during the campaign.

Without the help of people like Wendy in Iowa, who made 5,000 phone calls or the girls in Tulsa, Oklahoma, whose song "Game On" became an internet sensation, the campaign would not have come as far as it did, Mr Santorum said.

Mr Santorum mentioned his visit to the factory of the Minnesota manufacturer of his sweater vests, which became known as the former Senator's signature outfit.

'Worthy competitor'

Mr Santorum won a total of 11 primaries and caucuses , and picked up additional delegates in states that awarded them proportionally.

He emerged on the national scene on the night of the Iowa caucuses in January, eventually winning the state by a whisker after victory was initially handed to Mr Romney.

His old-fashioned, hard-working campaign style saw him visit every one of the Iowa's 99 counties in the months preceding the vote, and won him the respect and support of many in the state.

He continued to garner strong support in the Midwest and in the South, halting Mr Romney in a swathe of states from Minnesota to Alabama and as far west as Colorado and North Dakota.

In conceding that he could not win the nomination Mr Santorum made no specific mention of Mr Romney, and did not say whether he planned to endorse the front-runner.

However, he reportedly telephoned the former governor to concede shortly before speaking to reporters.

In a statement, Mr Romney congratulated Mr Santorum on his campaign, calling him an "able and worthy competitor".

"He has proven himself to be an important voice in our party and in the nation," the former Massachusetts governor said.

Gingrich and Paul defiant

Meanwhile, fellow candidate Newt Gingrich said Mr Santorum had run a "remarkable campaign", adding that "his success is a testament to his tenacity and the power of conservative principles".

Mr Gingrich, though, insisted that he would remain in the race in an effort to broaden the policy discussion and offer a conservative alternative to Mr Romney.

Texas Congressman Ron Paul congratulated the former senator on running a "spirited campaign". Mr Paul has the fewest delegates but, like Mr Gingrich, has refused to pull out of the contest.

Despite them remaining in the race, many analysts quickly characterised Mr Santorum's decision as the moment the general election campaign effectively began.

Mr Romney, who made his fortune in a private equity firm is now seen as the man to take on Democratic President Barack Obama, a former law professor and community organiser, in November's election.