Iowa Sen. Tom Harkin and Oregon Rep. Peter DeFazio, both Democrats, recently proposed a “Robin Hood” tax on trades of stocks, bonds and other financial instruments, saying it could raise $350 billion in the course of 10 years.

It’s a variation on a concept that’s drawing a lot of support from many important leaders – Microsoft Corp. Chairman Bill Gates, German Chancellor Angela Merkel and French President Nicolas Sarkozy, to name three.

From one angle, you could say that governments in general are just trying to cook up new sources of revenue because the economic climate is so troubled.

“We all agree that a financial transaction tax would be the right signal to show that we have understood that financial markets have to contribute their share to the recovery of economies,” Merkel told the German parliament.

From another angle, it seems reasonable to tax at least the most speculative transactions in all economic climates. If high-speed, high-volume traders and creative investment bankers were essential to the economy, we wouldn’t want to interfere. But they’re more destructive than essential.

According to The New York Times, both Great Britain and the United States are hesitant. The newspaper reported that the Obama administration says such a tax “would be hard to execute, could drive trading overseas and would hurt pension funds and individual investors in addition to banks.”

Certainly, there’s no point in taxing moves that are intended to provide financial security for individuals. But the riskiest maneuvers are all about seeking big scores, not security.

Legendary economist John Maynard Keynes in 1936 proposed a tax on Wall Street transactions as a way to tamp down excessive speculation and the resulting volatility. Since then, speculation has veered much further toward high-stakes gambling.

We have sin taxes, don’t we?