* Lagging in polls, Pawlenty, Huntsman go on offense
* Romney, Bachmann take more heat from rivals
By John Whitesides
WASHINGTON, July 19 (Reuters) - After months of criticizing President Barack Obama's economic leadership, the Republican White House hopefuls are turning some of their sharpest jabs on each other.
As the race for the 2012 Republican presidential nomination gains momentum, early front-runner Mitt Romney and rising challenger Michele Bachmann are taking heat from lagging rivals looking to narrow the gap and boost their profile.
While the tone is still relatively polite for an American political campaign, the intensity increased as the first state nominating contests in February 2012 draw closer.
"They are throwing sharper elbows because the pack has formed, it's clear who is doing well and it's time to start making a move," Republican consultant Ron Bonjean said.
"We're going into the next phase -- the introductions are over and it's time to start differentiating yourself," he said.
Tim Pawlenty, the former Minnesota governor who is languishing in single digits behind most of his rivals in opinion polls, has led the charge.
He has criticized Romney's support as Massachusetts governor for a state healthcare overhaul that was a model for Obama's federal law. He also accused Romney of ducking the debt ceiling debate, challenging him on his Twitter account: "What say you @MittRomney? Help us fight back."
Former Utah governor Jon Huntsman joined in, attacking Romney's record on job creation in Massachusetts. He said Republicans need a candidate who "actually has some credibility when it comes to job growth" to go up against Obama.
Romney, who leads most polls and raised more money than his rivals in the last three months, has largely tried to stay above the fray. His campaign said Romney led a sharp improvement in the Massachusetts job market.
The tension between Romney and Huntsman is certain to grow as they vie for more moderate, business-oriented Republicans, particularly in the early voting state of New Hampshire where both candidates need strong showings to sustain their bids.
The Republican pack of White House contenders has not excited many of the party's rank and file, giving an opportunity to late entries by other possible candidates such as Rick Perry, the conservative governor of Texas.
SHOWDOWN IN IOWA
In Iowa, which kicks off the nominating race, Pawlenty and Bachmann are vying for backing from the state's big bloc of social and religious conservatives. Polls show Bachmann, a U.S. congresswoman with a fiery speaking style, leading there.
Pawlenty criticized Bachmann's experience as he tries to make up ground.
"I like Congresswoman Bachmann. I've campaigned for her. I respect her, but her record of accomplishment in Congress is nonexistent," Pawlenty, previously known for a mild-mannered approach, said on NBC.
"We're not looking for folks who just have speech capabilities, we're looking for people who can lead a large enterprise in a public setting and drive it to conclusion," Pawlenty said.
The more aggressive approach is a turnaround for Pawlenty, who was criticized for ducking a chance during a New Hampshire debate last month to criticize Romney to his face for the healthcare law.
Pawlenty has used Twitter for some of his sharpest attacks on Romney, and to answer critics of his debate performance.
"On seizing debate opportunity re: healthcare: Me 0, Mitt 1. On doing healthcare reform the right way as governor: Me 1, Mitt 0," he said afterward on Twitter.
Former U.S. Senator Rick Santorum, another conservative looking for some traction, has taken a few swings himself. He released a Web video critical of both Huntsman and Romney for their refusal to sign a pledge opposing abortion rights.
The video lampoons the motorcycle-riding Huntsman, showing a man riding in the desert and then crashing as the words "hasn't signed the anti-abortion pledge. Just like Mitt Romney..." appear on the screen.
Like most front-runners, Romney has tried to stay out of the party squabbling and limit his criticism to critiques of Obama. If his grip on the race begins to slip, that could change.
"If you see Romney criticizing anybody besides President Obama, you know he is feeling threatened," Bonjean said.
(Editing by Will Dunham)