As America's biggest banks prepare to announce tens of billions of dollars in fresh writedowns from the international credit squeeze, they are turning to unlikely rescuers.

Sovereign wealth funds - or state-backed investors - are expected to buy further stakes in Merrill Lynch and Citigroup as the banks try to repair their battered balance sheets. The funds' role may be unnerving for some in Washington. But their involvement is as welcome as it will be necessary if the US economy is to avoid a prolonged slump.

A populist backlash against sovereign wealth funds may follow. Some US politicians, including Chuck Schumer, the influential New York senator, are worried that the biggest funds, owned by countries from China to the Gulf, could try to gain influence over US companies. They argue the funds are potentially susceptible to "non-economic" interests. So the closer they come to taking control, the greater the concern.

The main flaw in this argument is the absence of any proof to back it up. Because they lack transparency, there may be problems with the way sovereign wealth funds operate. But they have every financial incentive to be good stewards of the assets they buy. Their investment may even be an opportunity for US firms, as China's investment in Barclays, the UK bank, promises to be.