Former Massachusetts Gov. Mitt Romney still has to sell himself to conservative voters who have been looking for an alternative.
Many political experts believe Romney will ultimately be the Republican nominee to face Democratic President Barrack Obama next year but he has a long way to go yet.
Texas Gov. Rick Perry, a staunch conservative, has to prove he can hold his own on stage with his main rivals after four shaky debate performances that knocked him out of the front-runner position.
Cain, a former Godfather's Pizza chief executive and motivational speaker, has shot to the top of polls of Republican voters based on the simplicity of his "9-9-9" growth plan that would overhaul the US tax code.
Experts have questioned whether his idea to cut personal income and corporate taxes to 9 percent, while creating a 9 percent national sales tax, would in fact raise taxes on lower- and middle-income Americans, hurting the most needy.
Cain's economic plan, plus his absence of foreign policy credentials and no experience governing, may surface as a problem just as he begins to gain the attention of voters who find him a much-needed fresh face in politics with an inspirational style.
A longshot candidate, former Pennsylvania Senator Rick Santorum, hinted on Monday he may raise some questions about Cain's 9-9-9 plan.
"It's not as clean as he would like to make it out to be," Santorum told Radio Iowa. "I give him credit for bringing some innovative ideas but just because it's innovative and bold doesn't mean it's good."
"NOT PART OF THE PROCESS"
Jennifer Duffy, an expert at the non-partisan Cook Political Report, said Cain is the latest to benefit from conservatives looking for someone other than Romney because they have doubts about the depth of his conservative beliefs.
"Part of Cain's appeal is that he is not part of the process, he's not a politician," said Duffy. "He's a very plain-speaking guy ... He's the only one out there now who actually has a 'new' idea."
Conservatives had first been captivated by Minnesota Congresswoman Michele Bachmann and then Perry before flocking to the Cain camp, an example of the wide-open nature of the race to find a Republican nominee with less than three months to go until voting in primary elections starts.
Romney has had steady debate performances throughout and retains solid support in polls but conservatives have doubts.
"He needs to show he can continue to fight and be a good, strong conservative," said Republican strategist Scott Reed.
Perry has the task of recapturing the excitement when he first announced his candidacy two months ago. His debates, plus a position favoring college tuition assistance for illegal immigrants, has hurt him among conservatives.
"Perry needs to prove he can debate," said Duffy. "If he scores some points against Romney or Cain then he's going to have a good night."