The strength of the middle class has historically powered the U.S. economy. But now the middle class is suffering, and that means trouble for the economy, says MarketWatch columnist Rex Nutting.

A big part of the problem is household debt, he writes. "We've had a big debate about whether the nation can survive with a government debt-to-GDP ratio above 90 percent, but almost no discussion about what it means for private-sector debt to total 240 percent of GDP."

The economy is still too reliant on borrowing from the middle class, Nutting argues.

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"Recent data show that the middle class is once again borrowing, mostly for autos and education. Although the cost of servicing their debts has fallen to a record low thanks to low interest rates, middle-class families are vulnerable if interest rates rise significantly," he explains.

"And that means the economy is vulnerable. In order to grow, our economy requires spending by the middle class. But how can the middle class spend when their incomes are flat and they are already overburdened with debt?" Nutting asks.

"Unless middle-class incomes can rise along with productivity growth again, the U.S. economy probably is doomed to a long period of stagnation."

U.S. consumer credit soared by $26.85 billion in April to $3.18 trillion. That puts the annual consumer debt growth rate at 10.2 percent, the fastest pace since July 2011.

To be sure, not everyone is worried about the increase. "The ability of consumers to carry debt is vastly improved," Millan Mulraine, deputy head of U.S. research at TD Securities, tells Bloomberg.

"This is what we need to see for us to believe that the economy will make that transition to a self-sustaining growth trajectory."

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