Leonhardt defends his article:
I certainly agree that the two situations are not equivalent. Greece’s fiscal problems are worse than ours, and both our underlying economy and our political institutions are stronger than theirs. But the last statistic Mr. Krugman cites highlights why I think the comparison is relevant: “we have a long-run fiscal imbalance of 6-plus percent of G.D.P.” So to get our budget in order, we would need to come up with revenue equal to more than 6 percent of gross domestic product, either through tax increases or spending cuts. (That number comes from this paper, by the economists Alan Auerbach and William Gale.)
That’s an enormous amount of money. Military spending, for instance, is now less than 5 percent of gross domestic product. Medicare’s budget is now about 3 percent of G.D.P. Coming up with the necessary cuts and tax increases even over many decades, the relevant time frame will not be easy.