Economist Larry Summers expressed optimism about the economy's long-term prospects, telling CNBC that the first step in setting off a "virtuous circle" was to avoid the "fiscal cliff."
But that's a big if, he warned Friday on "Squawk Box."
With the economy growing but still vulnerable, Summers said, "if we didn't do something about the fiscal cliff … you're looking at an overwhelming likelihood of serious recession and you're looking at a real threat to national security."
Summers, who was Treasury secretary in the Clinton administration, said politicians need to prevent massive short-term damage to the economy and to put forward a longer-term framework to deal with the budget once the economy improves. (Read More: Why CEOs Are on the 'Fiscal Cliff' Warpath.)