Las Vegas ? 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 jump-starting 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.