The plan would do away with much of the current tax code and impose a 9 percent personal income tax, a 9 percent business tax and a 9 percent national sales tax, which tax experts say would mean that low- and middle-income Americans would pay more.

“Right now, we have a strongly progressive income tax. High-income people are paying a higher share of income in taxes than lower-income people,” said Alan D. Viard, a former Federal Reserve Bank economist and a resident scholar at the right-leaning American Enterprise Institute. “That is a pattern that would be disrupted by adoption of the Cain plan.”

The 9-9-9 plan has helped define Cain’s candidacy. Coupled with his buoyant, plain-spoken style, it has helped transform the former long shot into a front-runner. Cain has touted the proposal’s apparent simplicity and fairness, but he rarely delves into details. His campaign Web site shows that the plan is only a step toward achieving his ultimate goal: to eliminate the Internal Revenue Service after replacing all federal taxes with a national sales tax.

Meanwhile, analysts said the 9-9-9 part of Cain’s vision would place a further burden on those hit hardest by the nation’s economic problems.