A bigger issue is that a wealth tax may not be constitutional. The 16th Amendment provides only for an income tax. The estate, gift and generation-skipping taxes pass muster because they’re deemed excise taxes on the transfer of wealth, not on the wealth itself.

“Senator Warren’s proposal is based on a study by two scholars, and she’s pursuing that,” said Joanne Johnson, head of wealth advisory group at J. P. Morgan’s private bank. “There is a lot of extremely important analysis that still has to occur.”

A third issue is logistical. A wealth tax would be like an estate tax levied every year. Figuring out the tax owed on large estates is complicated, costly and time-consuming. The Internal Revenue Service gives estates a year to file a return, but even then, executors often have to file extensions. And on the other end, auditors go through the returns, which can take years before an estate is settled.

The process requires not just lawyers and accountants but valuation experts who assess the worth of assets like closely held family businesses.

“It would be a highly cumbersome tax return to prepare on an annual basis,” said Jeff Moes, executive vice president and chief fiduciary officer at FineMark National Bank & Trust, which serves high-net-worth clients. “Every federal estate tax goes through an audit, and presumably this would go through an audit as well. They’d have to figure out if the valuation methodology is correct.”

“A billionaire would have a return that would be literally three feet high,” he added. “Our $100 million clients own multiple closely held businesses. All of them would require an expert valuation and five-year financials.”

And then the government would need to have enough auditors to verify everything that was submitted. In 2018, for example, an estimated 4,000 estate tax returns will be filed, with tax owed on 1,900 of them. That’s a tenth the number of tax returns that would be filed under Ms. Warren’s wealth tax plan.