Representative Alexandria Ocasio-Cortez of New York, one of the most recognizable progressives in the House, and Senator Ron Wyden or Oregon, the senior Democrat on the Senate Finance Committee, are both working on mark-to-market bills.

Taxpayers would also be able to deduct any paper losses each year. Privately held assets would be exempt from the mark-to-market tax, but would pay an extra levy when they’re sold to minimize the incentive to defer a sale.

Wyden’s mark-to-market tax, for example, is aimed only at the very wealthy — those with at least $10 million in assets or $1 million in annual income for three years.

Taxing unrealized gains would almost certainly trigger lawsuits arguing that, for example, a gain in a stock’s price doesn’t qualify as “income” under the Constitution. But the U.S. tax code already uses a mark-to-market taxes on some financial instruments. Eliminating those rules would cause chaos, Glogower said.

A mark-to-market tax should be on more solid ground than a Warren-style wealth tax or Dutch-style levy, Glogower added. “You’re still actually taxing real gains or losses, and it’s just a question of timing.”

3. Make the wealth tax constitutional.

Many progressives – Warren and Sanders among them – prefer a simple wealth tax, which they insist is constitutional, even if other taxes -- such as levies on estates or financial transactions taxes -- would face fewer legal challenges.

The Supreme Court should be forced to rule on the matter, advocates say, and any tax legislation could include a fallback provision: If the wealth tax is struck down, the U.S. could move to some other option for taxing the rich.

One good fallback to a simple wealth tax, says Indiana University law professor David Gamage, is a wealth levy that is divided evenly among the states in a way that is almost certainly constitutional.

The disadvantage is that by requiring each state to produce the same revenue based on its population, the tax could require billionaires in poor states like West Virginia to pay much higher rates than the superwealthy in rich states like California.