Edwards lodged her criticism of the presumptive Republican presidential nominee's proposal Saturday at the annual meeting of the Assn. of Health Care Journalists.
Under McCain's plan, insurance companies "wouldn't have to cover preexisting conditions like melanoma and breast cancer," she said.
McCain has been treated for melanoma, the most serious type of skin malignancy. Edwards in 2004 was diagnosed with breast cancer, and announced a year ago that it had returned and spread into her bones, meaning it no longer could be cured.
McCain's plan focuses on offering new tax breaks for individuals who buy their own health insurance. But critics say the Arizona senator's proposal avoids giving insurers requirements on whom they must cover and how much they may charge.
His plan would make it difficult for people with preexisting conditions, but who aren't covered by a government- or job-sponsored plan, to buy individual coverage, Edwards said.
Cancer survivors are routinely denied insurance when they try to purchase it as individuals, health experts say.
Edwards also criticized McCain's proposal because it would allow companies to sell health insurance across state lines. The senator's campaign website says the effort would give consumers more options and promote competition throughout the healthcare system.
But Edwards said the plan would allow insurers to move their headquarters to states in which consumer protection laws are weak. Giving an example to back up her claim, Edwards noted that many credit card companies are based in Delaware, where the state's laws are more accommodating to corporate interests, she said.
"Hard-fought state-by-state protections would be lost," Edwards said. "They mask this proposal as a cost-saving technique. This is giving insurance companies a pass."
Douglas Holtz-Eakin, senior policy advisor to McCain, said Saturday that Edwards' comments were disappointing and that they revealed she did not understand the comprehensive nature of the senator's proposal.
Holtz-Eakin said McCain's policy would harness "the power of competition to produce greater coverage for Americans." Because McCain's plan would lower the cost of healthcare through competition, Holtz-Eakin said, it would reduce costs for consumers with or without preexisting conditions.
Regarding her own health, Edwards is "actually doing really well," she said. "The cancer is still there, but it's under control."
"Sen. Clinton's plan is a great plan" that closely resembles John Edwards' proposal, she said. Clinton's plan mandates that every American be insured. Elizabeth Edwards said only universal healthcare would resolve one of the problems plaguing the healthcare system -- its soaring cost.
"Until we get rid of the need for hospitals and other providers to cover the costs of people who are not covered . . . the overall cost is not going to go down," she said. "The only real cost savings comes when you have universality."
In North Carolina, meanwhile, John Edwards praised Sens. Clinton (D-N.Y.) and Obama (D-Ill.) in his first public speech since dropping his White House bid two months ago but declined to endorse either candidate.
"I have a very high opinion of both of them," he said at the Young Democrats of North Carolina convention. "We would be blessed as a nation to have either one of them as president."
The Associated Press contributed to this report.