Republican presidential hopefuls meet at Sands Expo Convention Center in Las Vegas on Tuesday for the eighth debate in their race for their party's nomination to challenge President Barack Obama in 2012.
Here is a look at the candidates:
Romney, 64, who lost the nomination race to John McCain in 2008, has remained at or near the front of the pack among the Republican presidential hopefuls for most of the campaign.
Romney, who co-founded private equity firm Bain Capital, has touted his business experience as a way to attack Obama's handling of the struggling U.S. economy. Critics say he was a corporate raider who cut jobs.
Romney stepped in to rescue the 2002 Winter Olympics in Salt Lake City after the games were tarred by allegations of bribery by top officials and were far behind revenue benchmarks. He brought in a new management team and cut costs.
While favored by pro-business Republicans, Romney is viewed skeptically by some conservatives because he was governor of liberal Massachusetts and is a Mormon, a religion some evangelicals do not consider Christian.
Republicans have attacked him because of a healthcare plan he helped develop in Massachusetts that became a model for Obama's healthcare law. Romney defends the state law and attacks the federal version, which he has promised to repeal.
A radio talk show host and former chief executive of Godfather's Pizza, the 65-year-old Cain was chairman of the Federal Reserve Bank of Kansas City's board of directors. Cain has overtaken Romney in some polls.
He has never held elective office and portrays himself as a political outsider. He ran an unsuccessful campaign for the Senate in 2004.
Boosted by strong debate performances and drawing attention for his 9-9-9 tax overhaul plan, Cain has jumped to the top tier of Republican candidates in opinion polls and won support of many in the conservative Tea Party movement.
Cain's signature plan would scrap existing U.S. tax codes for a flat 9 percent corporate, income and sales tax. Some experts say the sales tax portion of the plan could shift a heavier burden to lower-income earners.
Cain has yet to prove he can attract major donors and assemble the campaign team he would need to beat other Republicans.
The three-term Texas governor, 61, has lagged in recent polls after shooting to the top of the field when he jumped into the nomination race in August.
His lackluster performances in recent debates and several controversies have been hurting his prospects.
A social and fiscal conservative, Perry has come under heavy fire from Republican rivals for some relatively moderate immigration positions and an order that young girls in Texas be inoculated for a sexually transmitted virus.
Although he has never lost an election and is the longest serving governor in Texas history, his stumbling performance in debates has raised questions about whether he would stand a chance of defeating Obama next year.
Perry has stressed his economic record in Texas, saying his conservative leadership helped the state create more than a third of all new U.S. jobs in the past two years. His record also includes deep education cuts, low public service levels and high numbers of people without health insurance.
Perry has proven himself a formidable fund-raiser, reaching $17 million for the third quarter, despite only entering the race in mid-August.
An anti-war congressman from Texas who ran unsuccessfully for the party's 2008 nomination, libertarian Paul, 76, has for years pushed many of the positions that are now part of the Tea Party platform. His calls for steep cuts in the U.S. deficit and the size of government have moved to the mainstream.
A forceful debater, Paul has a dedicated following, raising $8 million in the third quarter of 2011. He receives a steady support of 8 to 10 percent in national opinion polls, but has not broken through to the larger electorate.
A former speaker of the U.S. House of Representatives, Gingrich, 68, was the main architect of the 1994 Republican congressional election victory and author of its Contract with America manifesto. He ended his 20-year congressional career after Republican losses in 1998 elections.
Leading members of Gingrich's campaign team resigned in June and he has had a poor showing in the polls.
Bachmann, 55, reached the top tier of Republican candidates after a strong performance in the first major debate in June. But she has since fallen to single digits in opinion surveys.
The Minnesota congresswoman won the Iowa straw poll in August and is now focusing her campaign in the state, although her support there has dropped to 10 percent or below.
Known for strong religious views and uncompromising positions on financial issues, Bachmann is seen as having little appeal to moderate Republicans or independents. She is leader of the Tea Party caucus in the U.S. House.
Santorum, 53, a former U.S. senator from Pennsylvania, made a name for himself opposing abortion rights and gay marriage while backing welfare reform. He has fought to enhance his profile in early voting states but remains far behind.
Huntsman, 51, is not taking part in Tuesday's debate. He resigned in April as Obama's ambassador to China to plan his presidential run.
Like Romney, Huntsman is a Mormon. The former governor of Utah and member of a wealthy chemicals family is a moderate, and he has not won over the conservative voters who play a big role in the nominating process. He is near the bottom of many national polls.