Eight Republican White House hopefuls meet in a nationally televised debate on Thursday, hoping to generate momentum two days before an Iowa straw poll that will test the strength of their campaigns.

The first Republican debate in nearly two months will give struggling contenders like Tim Pawlenty an opportunity to make an impression on Iowa voters before they deliver their verdict in Saturday's non-binding straw poll.

It also gives the rest of the pack a chance to directly confront front-runner Mitt Romney, who was not challenged at the last debate and has largely ignored his rivals to focus his campaign-trail attacks on President Barack Obama.

With less than six months remaining before Iowa holds the first presidential nominating contest in 2012, time is running short for candidates to begin making up ground.

"Given the placement in Iowa just ahead of the straw poll, you will see a lot of attention on this debate," said Donna Hoffman, a political scientist at the University of Northern Iowa.

"I think the campaigns are going to make a calculus that it is time to take on Romney," she said. "There will be a lot of people trying to get noticed and a lot of positioning on social issues."

The debate will be held at Iowa State University, site of Saturday's straw poll, an unofficial mock election that serves as an early gauge of the popularity and organizational muscle of Republican candidates in Iowa.

Looming over the debate is the likely candidacy of Texas Governor Rick Perry, who is expected to make his intent to enter the race clear on Saturday during an appearance in South Carolina that coincides with the straw poll.

Perry also will travel to the early-voting state of New Hampshire on Saturday and visit Iowa on Sunday in a blitz that will steal attention from the straw poll winner and signal his intention to run an aggressive and broad campaign.

COMPETITION FOR ROMNEY

Perry, a staunch social and religious conservative, stresses his strong job creation record in Texas. That could help him compete with Romney for the party's pro-business wing and make him a formidable challenger to Obama.

Romney got into a debating mood during a morning visit to the Iowa state fair in Des Moines, where he had a heated exchange with a group of hecklers who pressed him on whether the wealthy should pay more into the Social Security retirement system.

They shouted and chanted "Wall Street greed" as he tried to answer. "If you don't like my answer, you can vote for someone else," Romney said. "I'm not going to raise taxes, that's my answer. If you want someone who will raise taxes, vote for Barack Obama."

Romney said Social Security and the Medicare and Medicaid healthcare programs for the elderly and poor would have to be part of a long-term solution to the budget deficit.

Sarah Palin, the former Alaska governor and 2008 vice presidential nominee who has not ruled out a presidential run, was expected to visit the state fair on Friday along with a swarm of other contenders.

The debate, which begins at 9 p.m. EDT (0100 GMT on Friday), also will feature plenty of other story lines in a field of potential challengers to Obama that has not impressed some Republicans.

U.S. Representative Michele Bachmann will try to repeat her successful performance in New Hampshire's June debate and continue her strong showing in Iowa, where she leads polls.

Former Utah Governor Jon Huntsman, whose campaign has stalled after a much anticipated debut in June, will be making his first debate appearance on the national stage.

Huntsman is not participating in the straw poll or competing in Iowa, focusing instead on New Hampshire and Florida. Romney also is not participating in the straw poll but says he will compete in the state's caucuses next year.

Pawlenty, the former Minnesota governor who has lagged in single digits in polls in Iowa and nationally, faces a possible make-or-break test in the straw poll and the debate could be his last chance for a direct appeal to Iowa voters.

"He has to gin up some enthusiasm, and he just hasn't done it yet," Hoffman said.

Also participating in the debate will be former House Speaker Newt Gingrich, businessman Herman Cain, former Senator Rick Santorum and U.S. Representative Ron Paul.

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