Eight Republicans vying to replace President Obama will face off Thursday in a high-stakes debate in American's heartland that could separate the contenders from the pretenders.

The debate promises to answer many questions about the candidates. Can Rep. Michelle Bachmann repeat her winning performance from the last debate? How will former Utah Gov. Jon Huntsman do in his first national debate?

Can former House Speaker Newt Gingrich revive his flagging campaign after losing all of his braintrust? Will former Minnesota Gov. Tim Pawlenty attack his rivals, which he didn't do a last debate? How will former Massachusetts Gov. Mitt Romney handle the attacks likely to come from all sides as the front-runner?

Fox News, the Washington Examiner and the Iowa Republican Party are sponsoring the debate, which kicks off at 9 p.m. ET.

Watch the 2012 presidential debate Thursday live on Fox News and on FoxNews.com. Coverage will start at 8:45 pm ET at http://live.foxnews.com.

Earlier Thursday, Romney held his own in an exchange of tense words with protesters at the Iowa State Fair. A group of liberal activists with the Iowa Citizens for Community Improvement ambushed Romney, accusing him of wanting to "slash Social Security."

After being repeatedly interrupted, Romney raised his voice, telling the riled-up protester, "If you want to speak, you can. But right now, it is my turn to speak."

"I'll give you my answer. If you don't like my answer, you can vote for someone else," he said. "I'm not gonna raise taxes. If you want someone to raise taxes, then you can vote for Barack Obama."

Romney concluded by joking about the protesters, calling them "a few people in the front here."

"My guess is that they won't be voting for me. That's fine. They can vote for Barack Obama."

A Fox News Poll released Wednesday showed that Romney remains Republican primary voters’ preferred candidate.

But all risk being overshadowed by one Republican who won't be on the stage -- Texas Gov. Rick Perry, who will make "a definitive announcement that he is in the 2012 race for the presidency Saturday," Perry aides told Fox News. He will visit the early nominating states of Iowa, New Hampshire and South Carolina this weekend.

Coming off a national day of prayer for America forum last weekend, Perry is acceptable to most socially conservative Republicans worried about family issues such as abortion and birth control.

At least two declared candidates -- Bachmann and Pawlenty -- are hoping for a last-minute boost before an important weekend test vote that could make or break their campaigns.

Huntsman is looking to make a strong impression in his first debate, while Texas Rep. Ron Paul is out to prove his libertarian ideas are more mainstream than fringe.

Others struggling to gain traction -- Gingrich, ex-Sen. Rick Santorum of Pennsylvania and businessman Herman Cain -- are simply seeking to promote their issues and maybe even boost their profiles.

"This is the official opening of the season," said Republican strategist Terry Holt. "It's the debut and you better have your best stuff ready. That raises the stakes."

Romney, who lost the Republican nomination in 2008, is hoping for a repeat of a June debate in New Hampshire, where he emerged largely unscathed. He enjoys a commanding lead in fundraising and strong poll numbers and wants to protect both by sticking to his message.

"He will focus on jobs and why his 25 years of experience as a businessman and entrepreneur gives him a unique set of skills to lead on the economy," Romney spokeswoman Andrea Saul said.

Romney was a successful businessman who founded a venture capital firm before entering politics, a resume he can contrast with Perry, who has never held a private sector job and has held elected office or government positions for the last 27 years.

His top rivals are determined to deny Romney a chance to take on Obama. Pawlenty, whose performance in a previous debate was panned when he initially refused to repeat the "Obamneycare" epithet he coined to link Romney's health care law in Massachusetts with Obama's national health overhaul, looked to repair his image and start anew.

"This is Pawlenty's last chance," Republican strategist Rich Galen said. "If he doesn't do well, I suspect it's three strikes and you're out."

The other candidates are struggling just to gain their footing.

In recent weeks, Huntsman has seen a raft of departures from his campaign, including his campaign manager.

That shake-up has paled in comparison to the one Gingrich suffered when his entire brain trust resigned earlier this year.

Laden with debt and having trouble raising money, Gingrich has severely curtailed his campaign schedule and is not participating in the straw poll. Santorum, too, has faced fundraising and polling troubles but has worked the grass-roots activists in Iowa to build support among social conservatives.

Fox News' Serafin Gomez and The Associated Press contributed to this report.