The Democratic presidential candidate has proposed a 2% annual “ultra-millionaire tax” on net worth over $50 million and 6% on fortunes of more than $1 billion. Her campaign has estimated the measure would raise $3.8 trillion over a decade.

Several European countries have abandoned wealth taxes, in part because citizens have moved assets abroad. American supporters of the idea say it is easier to fight tax avoidance in the U.S., which taxes its citizens regardless of where they live.

The Penn-Wharton estimate doesn’t incorporate the beefed-up tax enforcement that Ms. Warren contemplates.

“You need a big investment in enforcement apparatus, which has been rather depleted—and Warren has got that in her plan,” said Simon Johnson, an economist at the Massachusetts Institute of Technology’s Sloan School of Management and an informal campaign adviser.