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When Sen. Elizabeth Warren (D) of Massachusetts proposed a wealth tax as part of her nascent presidential campaign, the idea immediately began drawing attention.

Most Americans back a wealth tax, new polls say. Economists are debating its pros and cons. There鈥檚 just one problem: It鈥檚 not clear it can fly.

There are constitutional challenges. Most of the developed world has moved away from wealth taxes because levies on wealth are harder to administer than those on income. And there鈥檚 a key question of motive: Do voters want to reduce inequality or the power of rich people over government? Or both?

鈥淚f you think it鈥檚 a war on plutocracy 鈥� you think that rich people have too much political power 鈥� then if these wealth taxes reduce the wealth of the very wealthy and reduce their political power, you鈥檙e satisfied, goal achieved,鈥� says Joel Slemrod, director of the Office of Tax Policy Research at the University of Michigan in Ann Arbor. 鈥淏ut if you鈥檙e thinking about this as a war on inequality [where] lower-income, lower-wealth people are not doing well, then you care about how much revenue you raise.鈥�

Senator Warren has espoused both goals, but from a practical standpoint it鈥檚 not clear which war her 鈥渦ltramillionaire tax鈥� is fighting.

In the United States, where income inequality has soared over the past three decades, wealth has become even more concentrated. During the 1980s, the top 1 percent of households owned 25 to 30 percent of the nation鈥檚 assets, according to Gabriel Zucman, an economist at the University of California, Berkeley. By 2016, they owned 40 percent of the assets. 听

So placing a tax on wealth and not just income holds lots of appeal to economists and politicians eager to reduce inequality. Warren鈥檚 plan would place a 2 percent annual tax on the wealth of Americans with $50 million or more in assets and a 3 percent tax on those with $1 billion or more.

Those rates sound low, but they are actually high, points out Alan Viard, a tax and budget policy expert at the American Enterprise Institute in Washington, D.C. For example, if someone owned bonds yielding 3 percent a year, then a 2 percent tax on the value of the bonds would amount to a 67 percent tax on the income from those bonds. A 3 percent wealth tax would wipe out all the income.

That鈥檚 before accounting for the effects of inflation or other taxes.

Denmark had a 2.2 percent levy on its wealthiest households 鈥� the highest rate of all the developed nations in the Organization for Economic Co-operation and Development (OECD) 鈥� until the late 1980s, when it cut the rate to 1 percent. In 1997, it abolished the tax completely, and the wealthy, especially the ultrarich, saw their wealth subsequently expand.

So if the goal is solely to reduce the wealth and power of the wealthy, then a 2 to 3 percent wealth tax might do the trick.

But capital is key to economic growth. People put their savings into savings accounts, stocks, or other financial instruments that create a pool of money that individuals and companies can use to fund business expansion or new ventures. Many economists warn that overly onerous taxes on capital would hurt growth.

For one thing, they could cause the wealthy to find loopholes to undervalue their wealth (especially hard-to-value assets like art), shelter their assets through legal loopholes, or even illegally hide their wealth by transferring it to offshore tax havens. In 2013, Professor Zucman estimated that 8 percent of the world鈥檚 household wealth was stashed offshore.

This risk of capital flight is a major reason many OECD countries have moved away from wealth taxes. In 1995, 14 nations taxed wealth; now there are four: Norway, Spain, Switzerland, and France. This year France replaced its wealth tax with a new wealth levy that specifically exempts financial assets.

A related complaint: Wealth taxes brought in less than expected. In Switzerland, for example, researchers found that for every 0.1 percentage point rise in the wealth tax, the amount of reported wealth went down by 3.5 percent. Other studies suggest this lowball reporting is less obvious in Sweden and Denmark, potentially because financial institutions in those countries report their customers鈥� holdings to the tax authorities; in Switzerland, they don鈥檛, says Marius Br眉lhart, an economist at Switzerland's University of Lausanne.

In the United States, 鈥渨e don't have an infrastructure for determining and monitoring taxable wealth,鈥� says Professor Slemrod of the University of Michigan. 鈥淚t鈥檚 not easy.鈥�

Then there are constitutional objections unique to the US. The Constitution bans any national 鈥渄irect鈥� tax that is not apportioned according to state populations. A wealth tax was not considered a direct tax until 1895, when an anti-populist Supreme Court used the language to strike down an income tax and effectively block a national tax on wealth. In 1913, the 16th Amendment made it legal to collect income taxes regardless of population but didn鈥檛 address wealth taxes. How today鈥檚 court might rule if such a measure were passed is not clear, constitutional scholars say.

None of this means that Warren鈥檚 wealth tax is dead on arrival. Quite the opposite. A new Politico/Morning Consult poll found that 61 percent of Americans (including 50 percent of Republicans) support a tax on the very wealthy, with just 20 percent opposing it.

Economists behind the plan expect it will raise $2.75 trillion in a decade, enough to fund a much more robust Internal Revenue Service as well as Warren鈥檚 priorities on child care, reduced student debt, and climate policies. To avoid the problems that OECD nations faced, Warren鈥檚 plan calls for a broad wealth tax with few if any loopholes and a hefty exit tax if rich Americans decide to give up their citizenship to avoid taxes.

鈥淵ou would see some people trying to hide assets abroad,鈥� says Dr. Viard of the American Enterprise Institute. 鈥淲e鈥檝e gotten better at tracking the foreign bank accounts. But there are other ways of hiding assets overseas.鈥�

Warren鈥檚 rivals for the Democratic presidential nomination have offered alternative ways to tax the wealthy. Sen. Bernie Sanders (I) of Vermont has proposed expanding the current estate tax, which also taxes wealth but only once: when it transfers to one鈥檚 heirs. And Rep. Alexandria Ocasio-Cortez (D) of New York has suggested a 70 percent income tax on people earning more than $10 million.

Given the rise of inequality at the top and prospects that it will continue, "there is a strong case to be made in the US and elsewhere for increasing the progressivity of taxation again," says Mr. Br眉lhart of the University of Lausanne.听And not just through a wealth tax, he adds. "Probably the most reasonable way of doing it is a little bit of everything."

[Editor's note:听This story was corrected to make clear that a small wealth tax could eliminate all the income from a bond portfolio and that the Supreme Court struck down a national income tax in 1895. The final paragraph was added after the story's initial publication.]