Ms. Warren would pay for the $20.5 trillion in new federal spending through a mix of sources, including by requiring employers to pay trillions of dollars to the government, replacing much of what they currently spend to provide health coverage to workers. She would create a tax on financial transactions like stock trades, change how investment gains are taxed for the top 1 percent of households and ramp up her signature wealth tax proposal to be steeper on billionaires. She is also counting on savings from cuts to military spending.

Ms. Warren’s estimate for the cost of Medicare for all relies on an aggressive set of assumptions about how to lower national health care costs while providing comprehensive coverage to all Americans. Like Senator Bernie Sanders of Vermont, she would essentially eliminate medical costs for individuals, including premiums, deductibles and other out-of-pocket expenses.

Critically, her new plan would not raise taxes on middle-class Americans, a question she has been asked over and over but has not answered directly until now. When confronted on the campaign trail and debate stage, she emphasized instead that her plan would result in higher overall costs for wealthy people and big corporations but lower costs for middle-class families.