Presidential hopefuls Rick Perry and Mitt Romney have clashed in the latest TV debate for Republican candidates.
Nine candidates took to the stage in Orlando, Florida, but the two current leaders hogged much of the limelight.
Mr Romney and Mr Perry sharply accused each other of flip-flopping on Social Security and healthcare.
Texas Governor Perry was booed as he said those who opposed his state's system of providing tuition to illegal immigrants' children were heartless.
Also appearing in Thursday's two-hour debate sponsored by Fox News and Google were Congresswoman Michele Bachmann, Georgia businessman Herman Cain and former Utah Governor Jon Huntsman.
Former House Speaker Newt Gingrich, former Senator Rick Santorum, Congressman Ron Paul and former New Mexico Governor Gary Johnson joined the fray.
Early in the debate, Mr Perry would not be drawn on declaring a jobs plan, instead touting Texas' economic performance.Mr Romney - who was criticised at an earlier townhall event for saying he belonged to the middle class, despite his vast wealth - refused to define his idea of rich and poor.
"I want everybody in America to be rich," he said. "I want people in this country to have opportunities."
The federal pension programme proved an early flashpoint, with Mr Romney taking aim at Mr Perry, who labelled the system a "Ponzi scheme" in an earlier debate.
Bearing in mind Florida's vast elderly population, Mr Perry said he advocated reform of Social Security, while adding that it should be safeguarded for existing beneficiaries.
But Mr Romney pointed to Mr Perry's past criticisms of Social Security.
"There's a Rick Perry out there that's saying that... the federal government shouldn't be in the pension business, that it's unconstitutional and to be returned to the states," said Mr Romney. "So you better find that Rick Perry and get him to stop saying that."
Mr Perry hit back, saying that Mr Romney had switched his position on healthcare between editions of a book he had published.
In one edition, Mr Perry said, Mr Romney advocated expanding the healthcare programme he signed in Massachusetts to the rest of the country.
"Then in your paperback you took that line out, so speaking of not getting it straight in your book, sir," said Mr Perry. "It's like badminton."Some spectators later jeered as Mr Perry defended the Texas system of giving in-state tuition to the children of illegal immigrants.
Mr Santorum accused Mr Perry of being "soft" on immigration, while Mr Romney said Texas' policy was equivalent to giving a $100,000 (£65,000) credit to those children.
But Mr Perry said: "If you don't think we should educate children who have been brought into our state by no fault of their own... I don't think you have a heart."
A poll released before the debate found Mr Perry with a 28-22% lead over Mr Romney among Florida Republicans.
But the survey by Quinnipiac University Polling Institute showed Mr Romney topped Obama 47-40 percent in a possible 2012 contest, with Mr Obama beating Mr Perry 44-42.
The contenders are vying for the Republican nomination to face Barack Obama in November 2012's elections.
The field is expected to narrow in the coming months as the Republican race heats up ahead of primary season early next year.