WASHINGTON -- A report released on the eve of the first presidential debate paints a stark contrast between President Obama's and Mitt Romney's healthcare policies and their potential impact on the uninsured and health spending.

The Commonwealth Fund said Tuesday 45 million more people would be uninsured under the proposed policies of former Massachusetts Gov. Mitt Romney compared with President Obama.

On the other hand, "When fully implemented, the Affordable Care Act (ACA) is projected to substantially reduce the number and share of adults and children who are uninsured in every state, in every income group, and in every age group," the report stated.

But the Romney campaign is discrediting the report, saying many of the assumptions used by Commonwealth are flawed.

The Commonwealth Fund, with the help of Massachusetts Institute of Technology economist Jonathan Gruber, PhD, modeled three scenarios: if the ACA were fully implemented, if it had never been implemented at all, and if Romney's proposals were to take effect. It estimated the number of insured, the cost of insurance, the impact on Medicare, and other outcomes.

Results were published Tuesday in the report, "How the Obama and Romney Plans Stack Up."

The report makes a set of assumptions on two key Romney proposals -- transforming Medicaid into a block grant program and providing tax advantages to people buying insurance on their own -- because details haven't been fleshed out.

Because of his turning Medicaid into a block grant system, Romney's plan would actually increase the number of uninsured by 12 million by 2022, the Commonwealth Fund study found. More than 80% of the increase in the uninsured stems from the cuts in Medicaid that would occur under Romney's proposal.

The Romney approach would cut spending for Medicare as well as Medicaid, the report noted.

"This approach to cost containment would shift the burden of growth in U.S. healthcare costs from the federal government to the states, to low-income families, and to Medicare beneficiaries, without addressing the underlying causes of rising costs," the authors wrote.

Nationally, the uninsured rate of nonelderly under the ACA would fall to 10% by 2022, the study predicted. Under Romney's policies, it would increase to 26%, and would be at 22% if the ACA were never passed.

The Romney plan would also hit moderate income families hard. The uninsured rate for that group of people -- defined as an annual income of $57,625 for a family of four -- would jump from 28.3% to 36.4%. Under the ACA, it would fall to 6.9%, the report stated.

While the ACA provides a tax credit ranging from $3,900 to $4,500 per person to purchase plans on the private market, Romney would provide a tax deduction ranging from $1,900 to $2,600. This would cause higher out-of-pocket spending under Romney's proposal.

A person would spend on average 18.1% of their income on health coverage in 2016 without the ACA, the Commonwealth Fund found. That same person would spend 9.1% of their income on healthcare with the ACA in place.

As for Medicare, Romney has offered to turn that program into a "premium support" system, where beneficiaries would receive on average $8,000 to offset to cost of purchasing insurance.

The Romney plan also would exhaust the Medicare Hospital Insurance (Part A) Trust Fund quicker -- by 2017 compared to 2024 with the law -- because it would end spending reductions and taxes and fees raised by the law. Repealing the ACA would increase Medicare spending by $716 billion by 2022.

To help control healthcare costs, Romney has offered to promote alternatives to fee-for-service payment models and cap noneconomic damages in medical malpractice lawsuits.

The Romney campaign bashed the report, saying it contributes little to the health reform conversation. The Commonwealth study "badly mistakes Gov. Romney's proposals. Worse, it assumes a fantasy world where Obamacare has been a success," Romney campaign spokeswoman Andrea Saul told MedPage Today.

Under the ACA, Americans have seen their insurance premiums increase, small businesses face massive tax increases, and seniors will have reduced access to Medicare services, she said.

"The simple truth is this: The American people do not want this law, we cannot afford this law, and when Mitt Romney is president he will repeal it and replace it with common-sense, patient-centered reforms that strengthen our healthcare system," Saul said.