As the ratio of its debt to gross national product eclipsed 100 percent and surged toward 150 percent, Greece has twice in the last two years nearly defaulted on its debt. Only successive bailout packagesfrom the European Union and International Monetary Fund prevented catastrophe.

When tolling up all the US debts , including huge unfunded liabilities to Social Security and Medicare, the US is on dangerous ground, Walker said in a CNBC interview.

"We are less than three years away from where Greece had its debt crisis as to where they were from debt to GDP," he said.

The US is nearing the 100 percent threshold which historically shaves about one percentage point off GDP, which was just 1.3 percent for the second quarter and 0.4 percent for the first quarter.

With the recent increase in the debt ceilingand continued higher budget deficits at the federal level, the US is on course for its own crisis, Walker said.

"We are not exempt from a debt crisis," he said. "We're never going to default, because we can print money. At the same point in time, we have serious interest rate risk, we have serious currency risk, we have serious inflation risk over time. If it happens, it will be sudden and it will be very painful."