During a 2-hour nationally televised debate in Las Vegas, all of Cain's rivals questioned the wisdom of the plan, while Cain urged viewers to visit his Web site and read the analysis posted there and "do their own math."
"None of my distinguished colleagues who have attacked me tonight understand the plan," Cain said.
Former Massachusetts Gov. Mitt Romney and former U.S. Sen. Rick Santorum of Pennsylvania both said middle income taxpayers will pay more in taxes under the Cain plan and former U.S. House Speaker Newt Gingrich of Georgia said the plan contains "much more complexities than Herman lets on."
"Change on this scale takes years to think through if you're going to do it right," Gingrich said.
As the debate shifted to healthcare issues, the other candidates -- notably Texas Gov. Rick Perry -- tried to put Romney on the defensive for reform during his term as governor of Massachusetts. Romney defended healthcare reform in his state against charges it is the basis for the Affordable Care Act healthcare reform law, which opponents of President Barack Obama call "Obamacare."
Perry also attacked Romney on illegal immigration, accusing him of hiring undocumented workers at his home and then lying about it. Romney said he had fired a company that had performed work on his home after he learned it had used workers in the country illegally.
During the exchange, Perry repeatedly interrupted Romney, who told the Texas governor he has "a problem with letting other people finish speaking."
"This has been a tough couple of debates for Rick," Romney said, referring to negative reviews form Perry's performance in recent GOP presidential debates.
Called on to explain apparently contradictory statements in recent days on whether he was joking or serious about building an electrified fence along the U.S.-Mexico border, Cain said he was serious about the fence.
"I believe we should secure the border for real and it would be a combination of the fence, technology, as well as possibly boots on the ground in some dangerous areas," he said.
Rep. Michele Bachmann of Minnesota said if she were president the United States would build a fence along the entire length of the border but Perry said "by the time that fence gets built there's a lot better way" to deal with immigration. Perry said the federal government "has failed miserably to defend and secure that border."
Romney has raised $7.5 million for his campaign from Wall Street while Obama has raised $3.9 million, an analysis indicated.
The third-most successful presidential candidate in the Wall Street money race was Perry, who has raised $2 million, the Center for Responsive Politics analysis said.
Obama has raised more overall than either Republican -- at $89 million, compared with Romney's more than $32 million and Perry's more than $17 million, the center said in a study.
The figures were released just ahead of the debate, hosted by CNN and the Western Republican Leadership Conference, with voters from 16 Western states and territories in the audience given opportunities to pose questions to the candidates. It is to be followed Wednesday by some GOP candidates speaking to a convention of Republican activists from across the West.
The debate was down one candidate from earlier forums. Former Utah Gov. Jon Huntsman Jr. withdrew in protest of Nevada's decision to hold its first-in-the-West caucus Jan. 14.
Romney, Perry, Cain, Bachmann, Gingrich, Santorum and Rep. Ron Paul of Texas decided to boycott the Nevada caucus but participated in Tuesday's debate.
On the eve of the debate, Paul said if he were president he would slash the federal budget $1 trillion in a single year and bring the budget into balance within three.
His budget plan would eliminate five federal departments -- Commerce, Education, Energy, Housing and Urban Development and Interior. It would abolish the Transportation Security Administration, leaving security at airports and other transportation systems up to the private sector, including the airlines.
His plan would end all war financing and foreign aid, remove 10 percent of the federal workforce across the board and slash congressional pay along with the president's annual salary, which would drop it to $39,336 from $400,000, he said.
A CNN/ORC International poll released Monday indicated Romney and Cain were statistically tied for the lead in the GOP race, with Rick Perry dropping to a distant third.
Twenty-six percent said they were likely to support Romney and 25 percent said they favored Cain. The 1 point difference was within the poll's 3 percentage point sampling error.
Cain had 9 percent support in September, while Romney had 22 percent.
Perry was favored by 13 percent, half the 26 percent he had last month, the Friday-Sunday telephone poll of 1,007 adult Americans indicated.
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