Here. He argues that down the road, taxes are going to have to rise over time to fund Medicare, and not just on the top 1%. He presumes that Americans will continue to support Medicare and demand that it stay in place. He's probably right. He mentions in passing that the top 1% pay 25% of the Federal taxes. Not income taxes, because that number is almost 40% in the latest available data. I assume he is accounting for payroll taxes.
Realism from David Leonhardt
By the way, the share of income going to the top 1% and the share of taxes paid are both probably going to be lower in 2008 and 2009 than in years past, because of the recession and the Wall Street layoffs.