Ben Bernanke, the boss of the powerful US Federal Reserve, is getting anxious.

Bernanke and his fellow policy-makers trimmed their 2010 growth forecasts at the Fed’s last board meeting in late June, warning that a weak job market would dampen growth. They also discussed the possibility that more monetary stimulus may be needed “if the outlook were to worsen appreciably”.

Bernanke’s mood certainly can’t have been helped by figures released overnight that showed US retail sales fell by a worse-than-expected 0.5% in June — the second consecutive monthly drop. David Rosenberg, Gluskin Sheff chief economist, and one of the first economists to point out the fragile underpinnings of the US economic recovery, argues that it’s hard to see how the US economy can escape a double dip.

In his latest newsletter, Rosenberg conducts a clinical dissection of the US economic recovery. He points out that since growth rebounded in 2009, the US economy has managed to grow at an average annual rate of 3.2%. Of this 2.1% — or about two-thirds — came from companies restocking their inventories. The remaining growth — a “paltry” 1.2% — represented real final sales. As Rosenberg points out, this is “the weakest post-recession recovery on record”.

What’s extraordinary is that the recovery is so weak despite the massive amounts of stimulus that it received — including government spending programs that pushed the government deficit to 10% of GDP and is causing the government debt-to-GDP ratio to rapidly head towards 100% of GDP. Other measures included the US Federal Reserve cutting official rates to near zero and an unprecedented tripling of the size of its balances sheet. There were also changes in accounting rules that allowed the financial sector to boost reported profit growth. And home buyers benefitted from mortgage rates at record lows, and access to easy and cheap financing from government agencies.

But despite all this stimulus, the US economy is bogged down. Sales of new motor vehicles are under pressure, now that the ‘cash-for-clunkers’ program has eased. And even though mortgage rates are at record lows, housing sales and applications for mortgages are hitting new lows, and there’s a massive backlog of unsold homes which are overhanging the property market, and putting pressure on property values.

“The consumer is not exactly rolling over, but spending fatigue seems to be setting in, along with a natural rise in the personal savings rate, whenever a quick fix fiscal policy gimmick runs its course and expires,” Rosenberg notes.

But things aren’t looking that much better in the corporate sector. Commercial construction is plagued by high and still-rising vacancy rates in the office and shopping centre space. And, Rosenberg notes, there’s little prospect of the US companies boosting their earnings by selling more offshore:

“It would be nice to see an export boom but the overseas economies, to varying extents, are tightening either monetary or fiscal policy to rein in growth. China is certainly not going to be in the same position it was back in late 2008 in terms of being a leader on the policy front that could ignite a power surge for the global economy.”

In fact, he points out the US trade deficit widened to an 18-month high in May. If US companies increased their capital spending, it will certainly benefit tech stocks, such as Intel. But capital spending only accounts for 7% of GDP and so could only have a marginal effect on overall economic activity

Finally there’s the government sector that represents 20%t of the US economy. Two-thirds of this represents state and local governments which are cutting services, laying off workers and raising taxes as they struggle to bring their deficits under control. And already 70% of the effects of the federal government’s stimulus spending have washed through the economy.

Rosenberg notes that the political mood in the US has turned against further stimulus, although this could change when the three million Americans on the unemployment roll lose their benefits due to the decision of US Congress not to grant a further extension. Then there are the tax cuts enacted by former President George Bush that are due to expire at the end of this year.

Overall, it looks likely that government spending cut-backs and tax rises will clip 1.5% off US GDP growth next year. It leads to Rosenberg’s big question: “If the peak inventory contribution is behind us, and all we have left is a baseline growth trend in real final sales of 1.2%, then how does the economy not contract in the coming year?”

It’s precisely this conundrum that has Bernanke so worried.