If Democrats win the presidency and Congress in 2020, they’re going to need sources of new revenue to take major new initiatives. This time around, Democrats are less likely to be constrained by concerns about balanced budgets than they were under Bill Clinton or Barack Obama. They don’t want to be put in the position of just cleaning up the mess left by Republicans, and they have a far better case for deficit financing of infrastructure and investments in human capital than Republicans did with their 2017 tax bill. But more generous social programs will require new taxes. Reversing the 2017 Republican tax giveaways to corporations and raising taxes on top incomes are the best ways of raising that revenue for the policies they campaign on in 2020.

Still, there are major policy and strategic questions about which of the progressive taxes to pursue. Warren’s wealth tax—2 percent annually on net worth above $50 million and an additional one percent on net worth above $1 billion—has the merit of being well targeted at the superrich, the class that has seen its wealth grow spectacularly while most families’ wealth has stagnated. It would help counteract the tendencies toward oligarchy in American society and politics.

But the wealth tax presents two kinds of problems that suggest Democrats might need to rely on the established federal taxes to raise revenue on a progressive basis. According to a traditional adage of public finance, “An old tax is a good tax.” A tax long in effect has well-established administrative mechanisms and legal foundations. That is the case for the income and estate taxes. The United States has successfully enforced both of them at high levels, and their constitutional grounding is secure. But neither of those things are true of a wealth tax.

Many countries have had difficulties carrying out wealth taxes, as a recent report from the Organization for Economic Cooperation and Development explains in detail. In 1990, 12 OECD countries had net wealth taxes, but by 2017 that number was down to four, and one of those (France) has since eliminated its wealth tax. “Decisions to repeal net wealth taxes,” the report observes, “have often been justified by efficiency and administrative concerns and by the observation that net wealth taxes have frequently failed to meet their redistributive goals. The revenues collected from net wealth taxes have also, with a few exceptions, been very low.”

Warren’s wealth tax is designed to meet these objections, and the economists consulted by Warren, Emmanuel Saez, and Gabriel Zucman, project that tax avoidance and evasion can be kept to a modest level. But they may be underestimating the ingenuity of the lawyers, accountants, and lobbyists who make up the wealth-defense industry.

Still, the administrative problems don’t seem to me dispositive; a wealth tax might be worth enacting even if its enforcement was incomplete. The more serious problem is the potential for a wealth tax to be overturned by the Supreme Court. In a recent law review article, Dawn Johnsen and Walter Dellinger make a compelling case for the constitutionality of a national wealth tax. Their view would almost certainly prevail if the Court did not have a far-right majority, but because it does, the Court might well strike it down.

So, if you’re in Congress and you agree that higher taxes on the rich are justified, do you vote for a wealth tax that might be overturned? Or do you work within the framework of the income and estate taxes to achieve the same purpose? It seems to me the answer is clear.

In the future, Democrats could come back to a wealth tax when liberals have a majority on the Court. They should try to avoid enacting that tax at a point when a conservative majority might establish a historic precedent against it.

In the long run, a generous system of social protection cannot be fully financed with taxes on the rich, especially if those taxes are successful in reducing extreme inequalities. The more egalitarian countries with extensive and stable welfare states rely on broad-based taxes, particularly value-added taxes, and eventually the United States will need to adopt a mechanism of that kind too. That, however, is an even bigger taboo. For now, it’s a good thing Democrats are talking about higher taxes on the rich.