Texas Governor Rick Perry's outspoken views on Social Security have come under fire as Republican White House contenders hold their latest TV debate.
Mr Perry labelled Social Security a "Ponzi scheme" and "monstrous lie" in last week's lively encounter.
This time he faced his rivals in Tampa, Florida - the US state with the largest proportion of elderly voters for whom pensions are a vital source of income.
A poll on Monday gave Mr Perry a wide lead over the other contenders.
Some 30% of Americans said they would support him to be the Republican nominee, compared with 18% for former Massachusetts Governor Mitt Romney, according to the poll by CNN and ORC International.
In Monday evening's fifth debate, hosted this time by CNN, Mr Romney went on the attack early against Mr Perry over Social Security
He challenged the Texas governor on whether he still believed an assertion in his book, Fed Up!, that the programme was unconstitutional.
Mr Perry, a conservative Tea Party favourite, countered that Mr Romney had labelled Social Security's finances - in his book, No Apology - as criminal.
"You've got to quote me correctly," Mr Romney responded. "What I said was taking money out of the Social Security trust fund is criminal and it's wrong."
The two rivals accused each other of "trying to scare seniors" with their rhetoric on entitlement.
The Texas governor also sought to reassure current retirees that their federal pension was "slam-dunk guaranteed".
Former US House of Representatives Speaker Newt Gingrich said he was not worried about Mr Romney and Mr Perry frightening seniors "when President Obama scares them every day".
Mr Romney also sought to cast doubt on Mr Perry's economic record as Texas governor, suggesting that he was merely lucky.
"If you're dealt four aces, that doesn't make you, necessarily, a great poker player," Mr Romney said, referring to Texas's Republican-dominated politics, oil wealth, zero income tax and conservative labour laws.Minnesota Representative Michele Bachmann - who has slipped behind since Mr Perry entered the race in August - also attacked him.
She said an attempt by the Texas governor to vaccinate schoolgirls against a sexually transmitted infection, HPV, had been "flat-out wrong".
Mrs Bachmann also suggested that a drug company, Merck, could have stood to benefit to the tune of millions of dollars.
"If you're saying I can be bought for $5,000, I'm offended," Mr Perry responded, citing a campaign donation from Merck.
Also participating on Monday were former Utah Governor Jon Huntsman, businessman Herman Cain, former Senator Rick Santorum and Texas Representative Ron Paul.
On the issue of immigration, Mr Perry was briefly booed as he defended his policy of allowing in-state tuition for some illegal immigrants in Texas.
He said it was better to let immigrants become contributing members of society, rather than "on the government dole".
Mrs Bachmann was cheered as she resumed the attack, saying: "I think the American way is not to give taxpayer benefits to people who are here" illegally.
Mr Huntsman also took aim at Mr Perry, for opposing a border fence, saying: "For Rick to say you can't secure the border I think is pretty much a treasonous comment."
Mr Santorum attacked Mr Paul over an "irresponsible" blog post he had published on Sunday, arguing that America's unpopularity in the Muslim world was down to its foreign policy.
Mr Paul stood by the blog, saying it was "just not true" that radicals wanted to attack America because it was free and prosperous.
He attracted possibly the loudest boos of the night when he made reference to al-Qaeda's complaints about US bases in Saudi Arabia and America's treatment of Palestinians.
The Tampa debate was co-sponsored by the Tea Party Express and more than 100 state and local Tea Party groups.
Mr Perry sought to explain his remarks on Social Security in an op-ed article in USA Today on Monday.
He said he did not envisage changes to the system that would affect current and soon-to-be retirees, but called for unspecified reforms.
Hours ahead of Monday's debate Mr Perry and Mr Romney both received campaign endorsements.
Mr Perry was backed by Louisiana Governor Bobby Jindal, while Mr Romney won the support of former rival candidate Tim Pawlenty.
The next debate will take place on 22 September in Orlando, Florida, ahead of the Florida Straw Poll, a non-binding vote that tests candidates' popularity among the party faithful.The field is expected to narrow in the coming months as the Republican race heats up ahead of primary season early next year, when the party will select its nominee to run for the White House in November 2012.