American equity investors have suffered a lost decade of portfolio performance — the S&P 500 is about where it was back in 1998 — and trillions of dollars of lost net worth, so it may seem a terrible time to hit them with a $100 billion-a-year investment tax. And, of course, it is.

But good sense isn’t stopping the AFL-CIO from pushing just such an ill-advised plan. The nation’s largest labor organization is proposing a tenth of a percent tax on every stock transaction, to fund infrastructure projects that would, presumably, employ union members.

It would be a version of economist James Tobin’s proposal to levy a tax on currency trades as a way of reducing speculation and volatility.

Anti-globalization forces later latched onto the idea. More recently, so too have financial reform proponents such as Adair Turner, the chairman of Britain’s Financial Services Authority, who recently suggested Tobin taxes may be a way of containing the size of the financial sector.

Now organized labor is genetically hard-wired to figuring out new ways to clobber capital, forgetting that America’s investor class resides on Main Street as well as Wall Street. But you might figure its political allies could be a wee bit savvier.

Yet congressional Democrats are also toying with similar ideas. Earlier this year, a group of House members introduced a bill that would impose a quarter percent tax on all securities transactions, with the hope of raising at least $150 billion a year.

More recently, one of that bill’s sponsors, Representative Peter DeFazio, an Oregon Democrat, has proposed a 0.2 percent tax on crude oil futures contracts to tamp down on speculation and pay for national transportation spending.

With a stock transaction tax, investors would get hit directly through passed-along trading costs, just as a cap-and-trade emissions plan on business would quickly mean higher energy costs for consumers.

But there would also likely be some indirect, though severe, impacts.

To the extent that active traders are nudged aside, volume would thin and spreads widen. Some investment firms might choose to locate offshore or trade overseas.

And if that happens, these various transaction taxes would generate less revenue than expected — as well as cost domestic jobs — perhaps prompting tax proponents to advocate even higher levies to make up for the shortfall.

And don’t forget that the Obama administration is already proposing to raise capital gains tax by a third for wealthier Americans.

Maybe if the unions are really looking to fund more infrastructure spending through higher taxes, they could show some sacrifice by supporting taxes on members’ “gold-plated” healthcare plans, whose benefit costs may be 50 percent higher or more than the plan of the typical American worker.

So far unions have been vetoing efforts in Congress to use just such a tax to pay for healthcare reform. Then again, raising taxes of any kind during a period of economic weakness is a risky proposition.