Talk about nasty. The bitter, face-to-face sniping at this week's Republican debate was just a prelude to the coming weeks as Mitt Romney's rivals seek to tear him down before the leadoff Iowa caucuses.

Increasingly on the defensive, Romney is being hammered on old issues _ like an accusation of hiring illegal immigrants to work on his yard _ and is creating new openings for everyone from Rick Perry to President Barack Obama.

"You won't hear a lot of shape-shifting nuance from me," Perry told Republicans gathered in Las Vegas on Wednesday, hitting Romney anew the day after the two sparred onstage during a debate. "I'm going to give the American people a huge, big old helping of unbridled truth."

The target was Romney, who over the years has reversed his positions on a series of issues that conservatives champion. And the sharper, more personal tone seems sure to shape the campaign in the next month as Perry looks to undercut the former Massachusetts governor's standing at the head of the pack.

Obama's team, too, wasted little time in going after Romney in personal terms.

"The core principle driving Mitt Romney? Getting elected," Obama campaign manager Jim Messina told reporters in a conference call.

Appearing unruffled at the attacks, Romney kept his focus on Obama and the economy on Wednesday, saying: "He should be less concerned about keeping his own job and spend more time helping the millions who are unemployed."

But more criticism against Romney is certain to come from fellow Republicans as the race for the GOP nomination enters a new phase and the 2012 general election inches closer.

For now, Romney tops state surveys and national polls, including the latest Associated Press-GfK survey, in the GOP campaign. Perry's and Romney's other rivals have mere weeks to change that dynamic before the Iowa caucuses on Jan. 3.

After five debates since Labor Day, the candidates won't meet again in that setting until mid-November. So they'll be mixing it up mostly from afar _ on the campaign trail, on the Internet and, soon, in television advertising by the candidates themselves as well as by Super PACs that are working on their behalf and can spend as much money as they raise.

The candidates will cross paths at a dinner in Iowa this weekend where they will try to court cultural conservatives who haven't yet rallied behind a single contender. It's a prime setting for candidates like Perry, Minnesota Rep. Michele Bachmann, businessman Herman Cain and others looking to emerge as the alternative to Romney. Iowa conservatives have long viewed Romney skeptically for his reversals on abortion rights and gay rights, and they have viewed his Mormon faith warily.

Perry also will give an economic speech on Tuesday in South Carolina. Romney contends his business background makes him the strongest Republican in the field able to take on Obama on the economy, and Perry needs to counter that. The Texan will point to his state's job growth during his tenure as governor, and, in a bid to win over fiscal conservatives, he plans to call for tax changes that would apply the same rate to all citizens, regardless of income level.

Previewing the proposal, Perry said Wednesday that jumpstarting the economy "starts with scrapping the 3 million words of the current tax code and starting over with something much simpler: a flat tax. I want to make the tax code so simple that even Timothy Geithner can file his taxes on time." It was a reference to Obama's Treasury secretary.

Behind the scenes, the candidates with the most money _ Perry and Romney _ and their allied groups are gearing up for the inevitable TV ad war. Each campaign is sitting on roughly $15 million and counting, and there are less than 75 days before the Iowa caucuses.

There's no doubt that the personal attacks that played out on stage Tuesday _ and that had been simmering behind the scenes for weeks between the Romney and Perry camps _ will now continue out in the open.

As GOP strategist Alex Castellanos said in a Twitter message after the most acrimonious debate of the year: "All the GOP candidates have lost their virginity now. Everybody attacks everybody from now on."

After the debate and again on Wednesday, the candidates' respective aides made that clear.

Romney adviser Ron Kaufman called Perry "a petulant little boy" and said that Romney "put him in his place."

"The governor of Texas came across as mean, petulant and nasty," Kaufman said.

Perry communications director Ray Sullivan suggested the sharper tone from Perry would continue, saying: "I suspect this tack will be part of future debates, will be part of the campaign, and that's probably good for the voters."

He explained Perry's sharper tone this way: "I think he was Rick Perry in his approach, and that was a good thing for us."

A more aggressive Perry showed up during the debate and quickly assailed Romney's character.

"Mitt, you lose all of your standing from my perspective because you hired illegals in your home, and you knew for about it for a year," Perry said, raising a topic that was an issue during Romney's 2008 presidential run. "And the idea that you stand here before us and talk about that you're strong on immigration is, on its face, the height of hypocrisy."

Romney defended himself, but in doing so gave critics an opening.

He said that he had told the company that worked on his lawn in 2006 and 2007 that all of its workers had to be in the country legally.

"I'm running for office, for Pete's sake, I can't have illegals," Romney said he told the company when he discovered that it employed illegal immigrants.

Obama's advisers jumped on that comment quickly, with Messina saying: "He didn't object to having undocumented immigrants working for him because it's illegal."

Democrats also cast him as out of touch with middle class Americans after he went to Nevada _ the state with the highest foreclosure rate in the nation _ and said he wants to allow home foreclosures to "hit the bottom" to help the housing industry recover.

There's no mystery as to why Obama's team is assailing Romney. Many Democrats see him as likeliest to win the GOP nomination, given his wide name recognition, his proven fundraising ability and his expansive campaign organization.

Democrats believe he could be a formidable contender, with his business background and economic pitch, against a Democratic incumbent trying to win re-election at a time of 9 percent unemployment.

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Elliott reported from Washington.