DES MOINES, Iowa - Republican Mitt Romney opened a broad and newly abrasive assault on rival Newt Gingrich on Thursday, dispatching surrogates and staff to cast him as unworthy of the GOP nomination and unfit to be president.

Romney is aiming to undermine his rising rival on both personal and professional fronts ahead of the 2012 campaign's opening contest Jan. 3 in Iowa - a reversal by the onetime front-runner who had previously all but ignored his Republican foes.

"He's not a reliable and trusted conservative leader because he's not a reliable or trustworthy leader," former Missouri Sen. Jim Talent, a Romney supporter, said of Gingrich, offering a preview of the attacks that Romney's team promised would continue in the next few weeks.

Romney allies also were giving him a boost, announcing a $3.1 million TV ad campaign in Iowa that is expected to include hard-hitting commercials against Gingrich.

Campaigning in South Carolina, Gingrich brushed off the verbal assault and insisted his campaign would not change its above-the-fray approach regarding fellow Republican contenders.

"We're focused on remaining positive," he said.

Gingrich's reluctance to engage may be out of necessity. He dramatically lags Romney in organizational firepower as he tries to rebuild his campaign after an early-summer implosion that left it deep in debt.

The onslaught of criticism from Romney and his allies, after months of focusing solely on President Obama and little on his GOP foes, comes as the race has developed into a two-person contest. Gingrich's quick rise in national and early-state polls threatens Romney's claim as the likeliest Republican to be chosen to challenge Obama next fall.

Romney's attack strategy carries risks.

If he's successful in tearing down his main rival, there's no guarantee that in a multicandidate field he will end up benefiting from a possible Gingrich fall. At the same time, congenial Iowa voters generally don't look kindly on candidates who engage in negative politics, and they could end up punishing Romney in a state that already presents hurdles for him. Plus, questioning Gingrich's adherence to conservatism could draw attention to Romney's own liabilities as having switched positions on key social issues including abortion and gay rights.

The candidate, himself, personally stayed out of the fray Thursday, raising money in private in Virginia.

But he's all but certain to weigh in with a sharp critique of Gingrich when he campaigns Friday in Iowa. His efforts there seemingly have boosted the importance of a state where Romney had worked to downplay expectations. Romney also will face Gingrich during a nationally televised debate in Des Moines on Saturday night.

TV ads against Gingrich are all but assured - either paid for by Romney's campaign or by the Restore Our Future political action committee, which is made up of staffers from Romney's failed 2008 presidential bid.

The group announced Monday that it was running a 30-second ad in Iowa that emphasizes Romney's private-sector experiences while castigating Obama's community organizing and academic background. A spot blistering Gingrich was in the works.

Romney's attack on Gingrich's ability to be president may resonate with Iowans.

A recent Washington Post-ABC News Poll in Iowa shows 43 percent of likely caucusgoers said Gingrich "has the best experience to be president," compared with 16 percent for Romney. They also trusted Gingrich more than Romney by big margins on handling the economy and immigration. The poll has a margin of error of plus or minus 4 percentage points for all potential voters.

Less than a month before the Iowa caucuses, Gingrich is running ahead of Romney in polls in Iowa.

Romney now faces a dilemma: Try to stop Gingrich with a full-throttle campaign in Iowa and risk falling short ahead of must-win New Hampshire or tentatively engage Gingrich in Iowa and risk allowing him to gain momentum heading into the first-in-the-nation primary Jan. 10.