Nearly six in 10 Medicare recipients would pay higher premiums under a hypothetical privatized system along the lines of what Republican presidential candidate Mitt Romney has proposed, according to a study released Monday.

The report by the nonpartisan Kaiser Family Foundation also found striking regional differences that could lead to big premium hikes in some states and counties. That finding instantly made it ammunition in the presidential campaign.

In the senior-rich political swing state of Florida, the hypothetical plan modeled by Kaiser would boost premiums for traditional Medicare by more than $200 a month on average. In Nevada, another competitive state, 50 percent of seniors would face additional monthly premiums of $100 or more for their coverage. A new pattern of regional disparities would emerge from overhaulingMedicare's payment system, the report said.

Romney and his running mate, Wisconsin Rep. Paul Ryan, have proposed changing Medicare to a "premium support" system dominated by private plans that are paid a fixed amount by the government. President Barack Obama says replacing the current open-ended Medicare benefit would shift costs to seniors.