A commenter correctly points out that McCain has proposed optional private accounts for Social Security, not wholesale privatization of the program. Quite right, but that doesn’t mean it’s a great idea. Here’s Christian Weller and David Madland from the Center for American Progress.
[Private accounts] also are financially irresponsible. Because privatization schemes divert money from the Social Security Trust Funds into private accounts, they require massive additional government borrowing to pay current beneficiaries, which decreases national saving and places additional burdens on future generations. Dean Baker of the Center for Economic Policy Research estimates that these costs could be as high as $200 billion a year (almost 2 percent of the U.S. Gross Domestic Product) over more than 30 years.