Sen. Elizabeth Warren has made a name for herself in the Democratic primary as a woman with a plan. Her liberal policy proposals are sweeping and often expensive, a fact that her rivals have seized upon as she has risen in the polls. And the policy that is intended to fund her vision is no less ambitious.

Warren wants to tax the country’s richest citizens on their accumulated wealth, not just their income. Instead of a more traditional approach to raising money from the rich, like hiking income tax rates, Warren’s wealth tax plan proposes a radical new approach to taxation that would target the fortunes of the nation’s wealthiest people by imposing a tax on all household assets — not just income — above the $50 million mark. And Sen. Bernie Sanders has even announced a more aggressive proposal that would cut some billionaires’ wealth in half in 15 years.

The idea of using a wealth tax to fund some of 2020’s bigger policy proposals is seductive. If it works, it could kill two birds with one stone — both curbing rising income inequality by redistributing some of billionaires’ wealth and providing a new source of tax money to fund social programs. It’s pretty popular, too, among Democratic primary voters. But while most Democratic candidates are united around the idea that the rich should be paying more in taxes, they’re not all convinced that a wealth tax is the best way to do it. Entrepreneur Andrew Yang has expressed skepticism about how it would work in practice, saying that it would have “massive implementation problems,” while Sen. Amy Klobuchar has said, “When I look at this, I think about Donald Trump,” suggesting that the plan might be too divisive for a general election.

How serious a problem are these criticisms for Warren? Could her rivals in the primary undercut one of her signature ideas by arguing that a wealth tax is the wrong solution to the problem of income inequality? There is a debate percolating among experts about whether her plan is practical or even constitutional. But there’s also disagreement about how fatal those flaws would be.

Part of the problem in trying to gauge the potential flaws of a wealth tax is it simply hasn’t been tried in the U.S. before, so we don’t know exactly how much money it would raise or how it would fare in the courts. Economists on Warren’s team have estimated that her wealth tax could raise $2.75 trillion over 10 years while Sanders’s camp has estimated their version would raise over $4 trillion in new revenue over the same span. That is quite a windfall — if they can pull it off. But even some left-leaning experts are skeptical it’ll raise that much money.

Additionally, a wealth tax would almost certainly face a legal challenge from well-funded conservative opponents. And it’s genuinely unclear whether it would ultimately be ruled constitutional. The issue isn’t that Congress can’t enact a wealth tax. It’s that if a wealth tax counts as a “direct” tax, Congress would have to ensure that the amount of money coming from each state was roughly the same on a per-capita basis, as there is a provision of the Constitution that bans direct taxes unless the amount collected is drawn equally from the states based on their populations. Given that wealth is not evenly distributed across the states, that equal distribution would be functionally impossible to ensure.

The fate of a wealth tax, then, would hinge on whether it counts as a direct tax. That’s a tough question to answer, because the Constitution itself doesn’t really define what a direct tax is, beyond the fact that the category includes a poll tax, which is a fixed amount charged for every person. Taxes like tariffs and certain others that can’t be fairly distributed on a per-person basis are generally not considered direct taxes. But how all of this would apply to a wealth tax isn’t entirely clear. The Supreme Court weighed in on this question more than 100 years ago — and not in the wealth tax’s favor. In 1895, the court struck down a federal income tax law because it taxed income generated from property, including land and other kinds of personal property, like stocks and bonds. The decision was controversial, and Congress and the states effectively reversed part of it 20 years later with the passage of the 16th Amendment which allowed Congress to tax income without worrying about how evenly it was distributed. But Congress’s authority to tax wealth wasn’t addressed by the amendment, and the Supreme Court hasn’t really returned to the issue in the past century.

Warren’s defenders argue, however, that the court simply got it wrong back in 1895, and that a modern wealth tax wouldn’t count as a direct tax. But the court’s right-leaning justices might approach the tax with a less favorable eye. And the existence of the old precedent could give the court’s conservative justices a way to dispatch a wealth tax relatively easily, which gives experts like Daniel Hemel pause. “A wealth tax could raise trillions of dollars — or, if it’s struck down by the Supreme Court, it could raise nothing,” said Hemel, a law professor at the University of Chicago. “That’s a really big risk if you care about the redistribution of income and you’re trying to figure out how to get it done.”

Then there are the critics who have argued that even if a wealth tax could survive a legal battle, it would be a nightmare to implement and might not raise as much money as Warren and Sanders have claimed. Yang, in particular, has homed in recently on the practical shortcomings of a wealth tax. In the October debate, he pointed out that many European countries tried wealth taxes of their own but eventually abandoned them, in part because they proved so difficult to administer.

True, the comparison between the U.S. and Europe may not be entirely apt. For one thing, many European millionaires simply moved to neighboring countries to escape the wealth taxes, which would be harder for Americans to pull off. But one lesson that probably would translate, according to Janet Holtzblatt, a senior fellow at the Urban Institute, is that a wealth tax would be difficult to enforce.

One central concern is that the rich, who are already good at evading taxes, would find new and creative ways to avoid paying taxes on their wealth. Part of the issue is that while income and certain kinds of wealth, like stocks, are easily verifiable, the value of a privately held company or a piece of art is genuinely hard to pin down. And the people targeted by the wealth tax also have the means to hire tax lawyers and accountants to sift through the law in search of loopholes. “I think a wealth tax would certainly raise a significant amount of money,” Holtzblatt said. “But will it raise enough money to pay for everything that’s been suggested? That’s a much harder question to answer.”

From a practical perspective, several experts told me that if the goal is to squeeze more tax revenue out of the rich, there is lower-hanging fruit politicians can go after. But the aspects of a wealth tax that make some tax professors and economists squirm — its newness and its focus on the super-rich — are likely part of what makes it so appealing to voters. A New York Times/SurveyMonkey poll conducted in July, for instance, found that two-thirds of Americans, including a majority of Republicans, support Warren’s proposal. Other tax plans floated by the candidates, like lowering the threshold for the estate tax or establishing a universal basic income, have received significantly less support in polls.

So if nothing else, the wealth tax has been a very successful symbol of the idea that the wealthy should pay their fair share. Sanders’s version of the wealth tax would affect more people and explicitly seeks to make billionaires an endangered species, which could make it more vulnerable to criticism. Warren, meanwhile, has so far taken pains to downplay the radicalness of a wealth tax, painting it as a modest strategy for tackling a broken tax system. But the political risks are still real for Warren, especially if her critics decide they want to focus on the details.