The dismal science, it seems, is an optimistic profession. Many economists, Roubini among them, argue that some of the optimism is built into the very machinery, the mathematics, of modern economic theory. Econometric models typically rely on the assumption that the near future is likely to be similar to the recent past, and thus it is rare that the models anticipate breaks in the economy. And if the models can’t foresee a relatively minor break like a recession, they have even more trouble modeling and predicting a major rupture like a full-blown financial crisis. Only a handful of 20th-century economists have even bothered to study financial panics. (The most notable example is probably the late economist Hyman Minksy, of whom Roubini is an avid reader.) “These are things most economists barely understand,” Roubini told me. “We’re in uncharted territory where standard economic theory isn’t helpful.”

True though this may be, Roubini’s critics do not agree that his approach is any more accurate. Anirvan Banerji, the economist who challenged Roubini’s first I.M.F. talk, points out that Roubini has been peddling pessimism for years; Banerji contends that Roubini’s apparent foresight is nothing more than an unhappy coincidence of events. “Even a stopped clock is right twice a day,” he told me. “The justification for his bearish call has evolved over the years,” Banerji went on, ticking off the different reasons that Roubini has used to justify his predictions of recessions and crises: rising trade deficits, exploding current-account deficits, Hurricane Katrina, soaring oil prices. All of Roubini’s predictions, Banerji observed, have been based on analogies with past experience. “This forecasting by analogy is a tempting thing to do,” he said. “But you have to pick the right analogy. The danger of this more subjective approach is that instead of letting the objective facts shape your views, you will choose the facts that confirm your existing views.”

Kenneth Rogoff, an economist at Harvard who has known Roubini for decades, told me that he sees great value in Roubini’s willingness to entertain possible situations that are far outside the consensus view of most economists. “If you’re sitting around at the European Central Bank,” he said, “and you’re asking what’s the worst thing that could happen, the first thing people will say is, ‘Let’s see what Nouriel says.’ ” But Rogoff cautioned against equating that skill with forecasting. Roubini, in other words, might be the kind of economist you want to consult about the possibility of the collapse of the municipal-bond market, but he is not necessarily the kind you ask to predict, say, the rise in global demand for paper clips.

His defenders contend that Roubini is not unduly pessimistic. Jeffrey Sachs, his former adviser, told me that “if the underlying conditions call for optimism, Nouriel would be optimistic.” And to be sure, Roubini is capable of being optimistic  or at least of steering clear of absolute worst-case prognostications. He agrees, for example, with the conventional economic wisdom that oil will drop below $100 a barrel in the coming months as global demand weakens. “I’m not comfortable saying that we’re going to end up in the Great Depression,” he told me. “I’m a reasonable person.”

What economic developments does Roubini see on the horizon? And what does he think we should do about them? The first step, he told me in a recent conversation, is to acknowledge the extent of the problem. “We are in a recession, and denying it is nonsense,” he said. When Jim Nussle, the White House budget director, announced last month that the nation had “avoided a recession,” Roubini was incredulous. For months, he has been predicting that the United States will suffer through an 18-month recession that will eventually rank as the “worst since the Great Depression.” Though he is confident that the economy will enter a technical recovery toward the end of next year, he says that job losses, corporate bankruptcies and other drags on growth will continue to take a toll for years.

Roubini has counseled various policy makers, including Federal Reserve governors and senior Treasury Department officials, to mount an aggressive response to the crisis. He applauded when the Federal Reserve cut interest rates to 2 percent from 5.25 percent beginning last summer. He also supported the Fed’s willingness to engineer a takeover of Bear Stearns. Roubini argues that the Fed’s actions averted catastrophe, though he says he believes that future bailouts should focus on mortgage owners, not investors. Accordingly, he sees the choice facing the United States as stark but simple: either the government backs up a trillion-plus dollars’ worth of high-risk mortgages (in exchange for the lenders’ agreement to reduce monthly mortgage payments), or the banks and other institutions holding those mortgages  or the complex securities derived from them  go under. “You either nationalize the banks or you nationalize the mortgages,” he said. “Otherwise, they’re all toast.”

For months Roubini has been arguing that the true cost of the housing crisis will not be a mere $300 billion  the amount allowed for by the housing legislation sponsored by Representative Barney Frank and Senator Christopher Dodd  but something between a trillion and a trillion and a half dollars. But most important, in Roubini’s opinion, is to realize that the problem is deeper than the housing crisis. “Reckless people have deluded themselves that this was a subprime crisis,” he told me. “But we have problems with credit-card debt, student-loan debt, auto loans, commercial real estate loans, home-equity loans, corporate debt and loans that financed leveraged buyouts.” All of these forms of debt, he argues, suffer from some or all of the same traits that first surfaced in the housing market: shoddy underwriting, securitization, negligence on the part of the credit-rating agencies and lax government oversight. “We have a subprime financial system,” he said, “not a subprime mortgage market.”