The dueling policy proposals from Ms. Warren and Mr. Sanders come as they compete intensely for support from the same universe of liberal voters and activists. Their tax proposals are helping to galvanize those on the left, but their ideas will also provide fodder for Republicans eager to paint them in a general election as tax-happy and too liberal.

Polls have found widespread support for the idea of taxing wealth. A poll conducted for The New York Times by the internet research firm SurveyMonkey this summer found that two-thirds of Americans, including a majority of Republicans, backed a 2 percent tax on households worth over $50 million, which is the heart of Ms. Warren’s plan.

The proposals for a wealth tax are among a variety of ideas for collecting more tax revenue that Democratic presidential candidates have put forward. On Monday, Mr. Sanders proposed a new tax on corporations that have a huge gap between what they pay their chief executive and their median worker. Jared Bernstein, who was chief economist for Vice President Joseph R. Biden Jr. from 2009 to 2011, said he has discussed with the Biden campaign a proposal to tax financial transactions.

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On the campaign trail, Ms. Warren explains the concept of a wealth tax by putting it in familiar terms, likening it to the property taxes that many Americans pay on their homes. For the superrich, she said in South Carolina on Saturday, “how about we include in yours not only your real estate, but also your stock portfolio, the diamonds, the Rembrandt and the yacht?”

Mr. Sanders is blunt about his desire to reduce the size of America’s biggest fortunes, even highlighting how much individual billionaires would have to pay in taxes under his proposal. He said last week that he did not believe billionaires should exist in the United States.

“There’s no question to my mind that the United States is moving toward an oligarchy,” Mr. Sanders said. “This is an issue that has to be addressed, and this wealth tax begins to do that.”