* First major debate shifts Republican race into high gear
* Candidates vie to stand out in slow-forming field
WASHINGTON, June 10 (Reuters) - U.S. Republicans Mitt Romney and Michelle Bachmann will be in the spotlight next week as the party's slow-forming presidential nominating race enters a new phase with its first major debate.
Nearly eight months before the first contest, Monday's nationally televised forum will give voters a chance to form early impressions of most of the top-tier contenders for the right to challenge President Barack Obama in 2012.
Seven White House hopefuls will participate, including three prominent candidates who skipped an early debate last month -- Romney, Bachmann and Newt Gingrich, who promises to be there despite Thursday's mass resignation of his senior staff.
The debate gives Romney, the marginal front-runner in an unfinished field, and Bachmann, an outspoken conservative who has not formally declared her candidacy yet, their first opportunity to test campaign themes on a national audience.
"It's a turning point. Up to now the only people paying attention have been activists and the media, and with this debate you get regular people starting to tune in and make first impressions," said Fergus Cullen, the former chairman of the New Hampshire Republican Party.
Four others who participated in a low-attendance first debate -- former Minnesota Governor Tim Pawlenty, U.S. Representative Ron Paul, former pizza executive Herman Cain and former U.S. Senator Rick Santorum -- also will be at the Manchester, New Hampshire, debate.
Romney, the former governor of Massachusetts, leads the Republican pack in most polls and his formidable fundraising has made him the uneasy front-runner. But the real competition at the debate could be among the other candidates, who will be vying to lead the party's anti-Romney wing.
"Who will emerge as the main alternative to Romney?" said Dante Scala, a political scientist at the University of New Hampshire.
"Will there be one of them who gives a performance that distinguishes them from the rest of the field?" he said.
AN OPPORTUNITY FOR BACHMANN
Bachmann could be a natural for the role. Her fiery attacks on Obama, Washington insiders and even her own party's leaders have made her a hit with cable news and conservative Tea Party activists.
She clearly stands out as the only woman in the field, but many Republicans have questioned whether she can form a credible national campaign.
Gingrich, former speaker of the House of Representatives, also faces skepticism about his future after staff desertions and a disastrous campaign launch that included an apology for criticizing Republican Representative Paul Ryan's budget plan.
Widespread dissatisfaction with the current field has led some Republicans to urge other big names to get into the race. Sarah Palin, the 2008 vice presidential nominee, and Texas Governor Rick Perry are among those still considering it.
Many party stalwarts already decided not to challenge Obama next year, when the White House's ability to control the campaign debate and raise more than $750 million in cash would make it a very difficult race.
The debate in New Hampshire, which holds the second nominating contest and could play a crucial role in the 2012 Republican nominating fight, will give candidates a chance to appeal directly to the state's influential activists.
Romney said on Thursday he will not participate in the Iowa straw poll in August, a key early test of organization strength in the state that kicks off the race. He is focusing his strategy on a win in New Hampshire.
Former U.S. Ambassador to China Jon Huntsman, another likely candidate who is expected to formally declare his candidacy soon, also has said he will largely skip Iowa to focus on New Hampshire and other states.
But Huntsman's decision to skip Monday's debate -- his campaign says he will not appear in debates until he officially enters the race -- has raised questions in New Hampshire.
"It's a missed opportunity, and it has people scratching their heads here," Cullen said. "Certainly everyone expects he will run, so why not do it? The upside outweighs any downside for him." (Editing by Eric Beech)