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"I don't see other examples anywhere else in the world where it has actually worked over a long period of time," Clinton said.
Clinton did say she's "all in favor" of reinstating the estate tax, which Republican lawmakers all but eliminated in their 2017 tax law.
Warren has proposed an "ultra-millionaire tax" of 2% on American households' net worth between $50 million and $1 billion and of 3% on households' net worth above $1 billion. Sanders supports a similar version that begins with a 1% tax on wealth over $32 million.
"I just don't understand how that could work, and I don't see other examples anywhere else in the world where it has actually worked over a long period of time," Clinton said at The New York Times' DealBook conference on Wednesday.
Clinton argued that it would be difficult to fairly evaluate Americans' net worth and would force many people to sell their assets to pay the taxes, since the superrich often hold their value in assets like stocks.
"If you were going to do a wealth tax and it was on assets ... how you would value it is, I think, complicated to start with," she said. "But assume you can get some system of evaluation, people would literally have to sell assets to pay the tax on the assets that they owned before the wealth tax was levied. That would be incredibly disruptive, so I think there are other ways to raise the revenues."
Clinton said she's "all in favor" of reinstating the estate tax, which Republican lawmakers all but eliminated in their 2017 tax law. Fewer than 2,000 Americans are expected to owe an estate tax this year.
Both Warren and Sanders support an estate tax in addition to their wealth taxes.
Over the past year, prominent voices on the left have pushed tax hikes on the wealthy and demanded that billionaires and corporations stay out of politics. Rep. Alexandria Ocasio-Cortez of New York has made headlines by arguing that it's "immoral" for billionaires to exist in a society with widespread poverty. The freshman congresswoman last year endorsed raising marginal tax rates on those who make over $10 million.
The proposals have ignited a heated conversation over taxation and inequality that will most likely keep economic populism at the center of the 2020 Democratic primary.