(CNN) -- GOP presidential front-runner Mitt Romney on Thursday aggressively defended his tenure as head of Bain Capital, asserting again that his private-equity career resulted in the net creation of over 100,000 jobs while also calling any job losses an unfortunate fact of life in the private sector.
Under criticism from some Republican foes -- as well as Democrats -- for his role in corporate downsizing in the past, Romney confronted the issue while campaigning in South Carolina, site of the next primary contest on January 21, and Florida, which holds its key primary 10 days later.
Romney seeks to parlay momentum from his victories in the first two states of the nominating process -- Iowa last week and New Hampshire on Tuesday -- into a knockout blow against his five GOP rivals by sweeping the upcoming contests in Southern states.
Conservative South Carolina has picked the winner of every GOP nomination fight since 1980, and Romney hopes his early victories and standing atop the polls in the state will bolster his bid to take on President Barack Obama in the November general election.
Speaking to reporters after a campaign event in the Palmetto State, Romney said that in the private sector "there are some businesses that are growing and thriving, and we were fortunate enough to be able to be a part of that in a small way."
At the same time, "there are some businesses that have to be cut back in order to survive," Romney continued. "... Sometimes we're successful at that, and sometimes we're not."
Romney said any job loss is a "tragedy," but insisted that every time Bain "invested in a business it was to try and encourage that business to have ongoing life."
He later appeared at a Florida event, telling a West Palm Beach audience that he believes in a "merit nation" where people achieve through education, hard work, risk-taking and "maybe a little luck."
"And by the way, as they were successful they did not make the rest of us poorer," Romney added. "Their success actually lifted the rest of us, as well."
Several of Romney's GOP rivals -- most notably former House Speaker Newt Gingrich and Texas Gov. Rick Perry -- are trying to halt the former Massachusetts governor's victory streak by hammering him on his private sector track record.
Perry, who recently castigated Romney for practicing "vulture capitalism" that generated wealth for a select few while gutting companies, pushed back Thursday against criticism from fellow Republicans that challenging Romney's business background amounted to opposing the free enterprise system.
"I love capitalism. Free market capitalism in the state of Texas has created over a million jobs," Perry said at an event in Orangeburg, South Carolina. "We understand how capitalism needs to work, but this corrupt and fraudulent activity going on in Washington, D.C., between them and Wall Street has to stop."
At a Columbia, South Carolina event, Gingrich expressed amazement "at the intensity of the counterattack," adding "it is almost as though if you ask questions you are somehow challenging the whole system."
"I think when you have crony capitalism or politicians taking care of their friends, that is not free enterprise," Gingrich said. "That's just back-door socialism in which the rich get all the money and the rest of us get all the debt. And I am not going to back down or be afraid to say that we the American people have the right to know and any candidate for president has an obligation to tell us."
The other Republican contenders -- Texas Rep. Ron Paul, former Pennsylvania Sen. Rick Santorum and former Utah Gov. and U.S. Ambassador to China Jon Huntsman -- have steered away from challenging Romney's venture capitalist experience.
An outside group backing Gingrich has promised to spend $3.4 million in South Carolina television ads largely blasting Romney's business record.
Strategists for the group -- Winning Our Future -- believe the ads will have particular resonance in a state with a reeling manufacturing sector and unemployment hovering close to 10%.
GOP critics of the ads characterized them as a form of class warfare better suited for a Democratic than a Republican campaign.
The president of the U.S. Chamber of Commerce, Tom Donohue, said Thursday that attacking Romney's tenure at Bain Capital was "foolish."
"We think Romney's had a pretty good track record. Perfect? Hell, no. But damn good," said Donohue, who heads the influential business advocacy group.
South Carolina Sen. Jim DeMint, a GOP powerbroker who has declined to endorse any of the candidate so far, defended Romney's record during an interview with MSNBC.
DeMint, who endorsed Romney in his unsuccesful 2008 presidential bid, called the focus on Bain Capital a "teachable moment." He also minimized Perry's and Gingrich's chances in the race, and said that "to damage a front-runner makes no sense to me."
In a later interview on CNN, DeMint said he would back whoever becomes the Republican nominee, adding all had good and bad points but any of the GOP contenders would be better than "what we have now."
Three prominent DeMint loyalists -- including a former top fundraiser for Perry -- were planning to publicly endorse Romney Thursday, a source confirmed to CNN.
One of the loyalists, former South Carolina GOP Chairman Barry Wynn, told CNN the escalating rhetoric about Romney's business background is "destructive."
"It's just a dance I didn't want to be a part of," Wynn said. "It kind of falls into this politics of envy that the Obama campaign is going to be all about between now and November."
Meanwhile, the pro-Romney group Restore Our Future announced the release of a new television ad Thursday ripping Gingrich's record. The ad, titled "Desperate," says the former speaker "has more baggage than the airlines." It focuses on Gingrich's past ties to mortgage giant Freddie Mac and his previous association with Democratic House leader Nancy Pelosi, among other things.
The group indicated it plans to spend $2.3 million airing the ad in South Carolina, along with $3.4 million in Florida, which holds its primary at the end of the month.
In addition, a group backing former Santorum is planning to buy $600,000 in new South Carolina ad time, a source familiar with the buy confirmed to CNN Thursday. The Red, White, and Blue Fund has already purchased $190,000 in ads touting Santorum's conservative principles.
Santorum's campaign has said it intends to spend at least $1.5 million in the state.
Desperate to regain momentum after a weak showing in New Hampshire following his close second-place finish in Iowa, Santorum challenged South Carolina voters to prevent Obama's re-election by choosing a true conservative like himself instead of a moderate such as Romney.
"If South Carolina gets it wrong, this country may be stuck with four more years of Barack Obama," Santorum said in Bluffton. "... We will be the generation that will have to explain to your children and grandchildren as to what you did, the opportunity you missed."
At another event in Hilton Head, Santorum raised Romney's well-known policy shifts on major issues including abortion and health care.
"Aren't you looking for someone as president you can trust?" Santorum said. "What would give you the idea that someone who's changed their position on nearly every single issue is a guy you can trust?"
Gingrich, meanwhile, has called on Romney to release his tax returns and identify major financial backers. Romney replied he has complied with all legal requirements so far, but left open the possibility of disclosing more information in the future.
Romney has spent more than $3 million on his campaign so far, and a super PAC backing him has spent another $2 million.
The total is almost $1 million more than the amount spent by Perry and a super PAC supporting him, the closest competitor on the list. However, other super PACs helping conservative candidates -- and therefore not Romney -- have spent more than $13 million so far.
Fresh off his strong second-place showing in New Hampshire, Paul's campaign plans to spend about $1 million in South Carolina, said campaign manager Jesse Benton.
Paul's aides admitted his organization in Iowa and New Hampshire far exceeded the campaign organization in South Carolina and beyond. They hope his third-place finish in Iowa and strong runner-up performance in New Hampshire will spur grassroots growth.
Even if Romney is unstoppable as the GOP nominee, Paul and his aides made clear he intends to keep his campaign going, perhaps all the way to the convention. The more delegates he can rack up, the more leverage he would have to integrate key messages of his libertarian, anti-interventionist movement into the Republican Party platform.
CNN currently estimates that Romney has won 25 GOP national convention delegates, compared to 10 for Paul, eight for Santorum, four for Perry, three for Gingrich, and two for Huntsman.
1,144 delegates are needed to win the Republican nomination.
CNN's Alan Silverleib, Dana Bash, Peter Hamby, Paul Steinhauser, Robert Yoon, Tom Cohen, Shawna Shepherd and Rachel Streitfeld contributed to this report