It’s important to bear in mind (1) that credibility is a function not only of expertise, but also of objectivity and sincerity, and (2) that Krugman is first and foremost an ideological hyperpartisan willing and eager to use his expertise to mislead people to support policies he prefers.

Here's what Krugman said in 2003 amid debate over the 2003 Bush tax cuts (to which he objected, of course, and which were passed two months later) in a column entitled "A Fiscal Train Wreck" http://www.nytimes.com/2003/03/11/opinion/11KRUG.html :

it's clear that the 10-year deficit will be at least $3 trillion...we're looking at a fiscal crisis that will drive interest rates sky-high...what's really scary... is the looming threat to the federal government's solvency. That may sound alarmist: right now the deficit, while huge in absolute terms, is only 2 , make that 3, O.K., maybe 4 percent of G.D.P. But that misses the point ... because of the future liabilities of Social Security and Medicare, the true budget picture is much worse than the conventional deficit numbers suggest.

... the conclusion is inescapable. Without the Bush tax cuts, it would have been difficult to cope with the fiscal implications of an aging population. With those tax cuts, the task is simply impossible. The accident -- the fiscal train wreck -- is already under way.

In other words, Krugman is saying in 2003 that the projected long-term fiscal imbalance at that time should be associated with an extremely high probability of great harm in the long term. As of 2003, with the Bush tax cuts, "coping" with our fiscal future is "simply impossible" and the "fiscal train wreck is already underway".

Flash forward to 2009, as Krugman favors more deficit-spending as stimulus (and adding a new healthcare entitlement that will make it tougher to solve our long-term fiscal imbalance problem). The projected long-term fiscal imbalance is now much WORSE from any perspective: the projected deficit numbers themselves, timing (baby boomers start retiring now), drivers of the imbalance, and political, economic, and lifestyle difficulty of solving the problem. Yet "Krugman 2009" characterizes as "hysterical" those who express anywhere near the level of concern that Krugman 2003 expressed. Krugman 2009 writes:

There’s been some hysteria about the administration’s new estimate that the cumulative deficit will be $9 trillion over the next decade. Don’t get me wrong: this is bad. But it’s being treated as an inconceivable sum, far beyond anything that could possibly be handled. And it isn’t.

... Right now, federal debt is about 50% of GDP. So even if we do run these deficits, federal debt as a share of GDP will be substantially less than it was at the end of World War II. It will also be substantially less than, say, debt in several European countries in the mid to late 1990s.

Again, the debt outlook is bad. But we’re not looking at something inconceivable, impossible to deal with; we’re looking at debt levels that a number of advanced countries, the US included, have had in the past, and dealt with.
http://krugman.blogs.nytimes.com/2009/08/23/how-big-is-9-trillion/

Even leaving aside the absurdity of his comparison of our current long-term fiscal challenge with our post-WWII experience, I presume that in 2003 Krugman was already very familiar with the fiscal history of the post-WWII years and at least with the experience to that point of those European countries with high debt levels in the mid/late-1990s, so it's not like Krugman has experienced some revelation that would explain his marked shift in the degree of long-term harm/risk he associates with a given level of projected long-term fiscal imbalance. I posted comments on a few threads of his blog asking him what accounts for this difference, never with any reply.

Krugman is a quintessential example of a hyperpartisan expert, one who abuses his credibility as an expert to mislead people on analytical matters in his advocacy for policies he prefers (and to feed red meat to his hyperpartisan fan club). Rather than presenting trade-offs in good faith, making a case for why one set of trade-offs (and related policies) are preferable to another, and leaving each reader to apply his own values and priorities in deciding for himself which trade-offs (and thus which policies) best reflect his values and priorities, he misleads people about what those trade-offs are.

H/T to Jim Glass for bringing those two Krugman column to my attention via blog post elsewhere.

See also my post last December at http://swordscrossed.org/diary/20081202/nobel-prize-winning-economist-de...

"fact isn't likely to change many minds."

I guess you've been a Republican for a while. There is another party in this country that takes a relatively grown-up approach to these issues you know.

Like many pundits he writes too often. If he stuck to his areas of expertise, such as international trade, he'd lose his perch at the NYT and his fan base but might enjoy a better reputation for intellectual honesty. The big stage has cost him something.

The question how much budged deficit is tolerable can not be answered in an objective way, because for a very long time it is predominately a psychological question.

As long as no suspicion arises and the big financial players accept it, the deficit funded party may go on...