Herman Cain has succeeded in introducing himself to voters. He has moved from the bottom of the pack all the way to the top. His “9-9-9” tax plan is the most talked-about proposal of the fight for the Republican nomination.

But as the battle moves beyond televised debates and into a phase in which organization and on-the-ground campaigning become increasingly crucial, Cain will face his toughest test yet — demonstrating that there is more to his tax plan, his campaign structure and his positions on other crucial issues than he’s shown so far.

As questions add up over what the 9-9-9 plan would do, whom it would help and hurt, Cain’s explanations have fallen short of answering them. The essence of his response is: I’m right and you’re wrong. At the same time, he must contend with other weaknesses in his campaign, including his stumbles on foreign policy and lack of organization in crucial states.

“There are a lot of moments where he says ‘I misspoke’ or ‘I was joking’ or ‘I didn’t mean that,’ ” said Chuck Laudner, a former executive director of the Iowa GOP. “It’s a worry with a lot of caucus-goers that I’ve talked to.”

“Everybody likes Herman Cain, so they’re not being very harsh with him,” Laudner added. “But they do understand that he has to come out here and explain the details and has to have a firm grasp on these issues. Everybody’s in crisis mode no matter what you’re looking at, from the war on terror to housing to Social Security. They want to make sure the candidate they’re going to vote for has a handle on these issues.”

Cain said in a television interview after Tuesday night’s debate that he continues to build his operation in key early states, but when pressed he acknowledged that he has just three people on the payroll in New Hampshire, home of the nation’s first primary. Cain’s campaign is also struggling to keep up with the onslaught of attention his 9-9-9 plan is drawing; during the debate, his Web site crashed after he urged viewers to go there for his explanation of how it would work.

Cain aides have said they are ramping up efforts to take advantage of the candidate’s newfound popularity.

“Our staff has grown to over 40 in the past week and continues to rapidly expand at the national headquarters level and in early primary states,” Cain spokesman J.D. Gordon said Wednesday.

The volume of people who want to learn more about Cain speaks to his appeal in a year when Republican voters have careered from one front-runner to the next. Cain’s brimming confidence on the debate stage and his sunny appeal on the campaign trail have helped elevate the stature of a candidate who was known by fewer than 20 percent of Americans a few months ago.

It is crucial for Cain to take advantage now, Laudner said, because the field finally seems set.

“Iowa caucus-goers are looking around and saying, ‘Okay, I have to make my choice,’ ” he said. “So the phone lines are burning. I’ve been on the phone constantly.” He added that he has seen no evidence of a Cain operation in Iowa.

Cain’s tenuous position was clear during the CNN-sponsored debate Tuesday, when some of the more dramatic moments of the evening featured not him but his two leading rivals, Texas Gov. Rick Perry and former Massachusetts governor Mitt Romney.

Cain also stumbled on a question on foreign policy, saying in an earlier interview with Wolf Blitzer that he would negotiate with al-Qaeda over the release of prisoners at Guantanamo Bay but then denying to debate moderator Anderson Cooper that he had said it. Then, when shown the video clip, he said, “I misspoke.”

Cain was pressed by Cooper, as well as his opponents, on his tax plan. Asked to respond to a new independent analysis showing that the plan would raise taxes on the vast majority of Americans but cut taxes for most millionaires, Cain called the criticism a “knee-jerk reaction” and said, “That simply is not true.”

The 9-9-9 proposal would wipe clean the tax code and replace it with 9 percent taxes on individual and corporate income as well as sales. The plan for a national sales tax has attracted particularly intense criticism.

During the debate, Cain shrugged off the concern about adding a new national sales tax on top of many existing state taxes, describing the two as “apples and oranges.”

“Fine,” Romney rejoined. “And I’m going to be getting a bushel basket that has apples and oranges in it, because I’ve got to pay both taxes.”

Cain was not helped by an interview this month with one of the architects of his plan, economist Stephen Moore, who told radio host Larry Kudlow that Cain should drop his proposed national sales tax.

“I’m surprised how hostile people are to the sales tax,” Moore told Kudlow. “When we designed this plan, I thought people would go along with the 9 percent sales tax. But the point is, they won’t.”