DES MOINES, Iowa - Attacked as a lifelong Washington insider, newly minted Republican front-runner Newt Gingrich parried criticism from Mitt Romney in campaign debate Saturday night, telling the former Massachusetts governor, "The only reason you didn't become a career politician is because you lost to Teddy Kennedy in 1994."

Gingrich also defended against attacks from Texas Rep. Ron Paul and Minnesota Rep. Michele Bachmann in the first debate since he soared to the lead in polls nationally and in Iowa. Caucuses on Jan. 3 in this state will kick off the competition for Republican National Convention delegates who will pick an opponent to President Barack Obama.

Under questioning from Paul, Gingrich said he had never lobbied for Freddie Mac, a quasi-government agency that paid him at least $1.6 million to provide strategic advice. Paul shot back, "It's the taxpayers' money, though. We were bailing them out."

The tone was respectful, at least in the early moments of the debate, the stakes ever higher as six rivals met on a stage in the Iowa capital city. The debate was the 12th since the long campaign began, and the first since Herman Cain's candidacy imploded after allegations of sexual harassment and an extra-marital affair.

The six contenders split down the middle on legislation making its way toward a year-end vote in Congress to extend a Social Security payroll tax into 2012.

Romney, Gingrich and Paul said they favored it. Bachmann, Texas Gov., Rick Perry and former Pennsylvania Sen. Rick Santorum said they opposed the measure.

Whatever the impact of their differences on the presidential race, the internal disagreement could well portend difficulties for legislation that Obama has proposed and Republican leaders in Congress view as essential if the party is to avoid being tagged for raising taxes.

For Gingrich, the debate brought new standing - a center position onstage that comes with being a leader in the polls - as well as the challenge of fielding criticism from his rivals.

Paul has been airing television commercials in Iowa attacking the former House speaker, and Romney's campaign has become increasingly critical of him, bolstered by a multimillion-dollar television ad campaign that is financed by allies.

Bachmann criticized Gingrich for first supporting a requirement for individuals to purchase health insurance in 1993. She folded Romney into her attack, saying he had pushed successfully for a state health care law as Massachusetts governor that contained an individual mandate.

In rebuttal, Gingrich said the mandate he supported was a conservative alternative to President Bill Clinton's national health care plan.

"It's now clear the individual mandate is unconstitutional," he said.

Gingrich's decision to invoke Kennedy, the late senator from Massachusetts, served as a dual reminder - that Romney has been running for office since the mid-1990s and also that he lost to the man whose politics conservatives detested above all others.

As for the question of whether he would have become a career politician if he'd beaten Kennedy, Romney replied with a smile that if he'd been good enough to play in the National Football League, as he had hoped, "I would have been a football star all my life, too."

He then tried to turn the tables, saying his defeat in 1994 "was probably the best thing I could have done for preparing me for the job I am seeking, because it put me back in the private sector."

One of Romney's campaign calling cards is his career as a businessman, a time he says helped him understand how jobs are created.