In his model, Gruber studied the effects of the 2010 law as opposed to repealing it and replacing it by providing states with Medicaid block grants and new incentives to purchase individual coverage, two proposals from Romney.

The study shows the Affordable Care Act is estimated to lower the uninsured rate to 10 percent from 15 percent in 13 states across the South and West, and also New York and Washington D.C. The law is expected to lower the uninsured rate to less than 10 percent in the remaining states.

Under Romney's plan, 30 percent or more of the under-65 population in nine states—Arkansas, California, Florida, Louisiana, Mississippi, Nevada, New Mexico, South Carolina and Texas—would be left uninsured by 2022, which the report says would increase the number of uninsured in every state. The report studied the candidates' positions in seven areas: health insurance coverage, insurance affordability, consumer protection, consumer choice, providing help for small business, improving Medicare, and improving healthcare quality and slowing healthcare spending growth.

Meanwhile, Grace Marie Turner, president of the Galen Institute—a conservative, not-for-profit research organization that focuses on health and tax policy—argued in her recent analysis for FoxBusiness that the health reform law will end up harming the nation's poor and senior populations.

"Instead of reforming Medicaid, the Obama administration is encouraging states to add as many as 16 million more people, crushing those already enrolled beneath a mountain of new enrollees and bringing Medicaid enrollment to nearly 85 million in 2020," Turner wrote, adding that the country's physician shortage exacerbates the problem, as a recent survey from the Physicians Foundation showed doctors will either stop seeing or restrict their number of Medicaid patients.