WASHINGTON — Elizabeth Warren on Friday proposed $20 trillion in federal spending over the next decade to provide health care to every American without raising taxes on the middle class, a politically risky effort that pits the goal of universal coverage against skepticism of government-run health care.

The details of Warren’s “Medicare for All” plan aim to quell criticism that the Massachusetts Democrat and presidential candidate has been vague about how she would pay for her sweeping proposal. Her refusal to say until now whether she would impose new taxes on the middle class, as fellow progressive White House hopeful Bernie Sanders has said he would, had become untenable and made her a target in recent presidential debates.

However, her detailed proposal was quickly attacked by her moderate rivals, including former Vice President Joe Biden. And some independent experts questioned whether her numbers were realistic.

In a 20-page online post, Warren said her plan would call on employers to transfer to the government almost all the $8.8 trillion she estimates they would otherwise spend on private insurance for employees.

“We can generate almost half of what we need to cover Medicare for All just by asking employers to pay slightly less than what they are projected to pay today, and through existing taxes,” she wrote.

Companies with fewer than 50 employees that don’t already sponsor coverage would be exempted from the proposal. And in a nod to unions whose support will be key in the Democratic primary, Warren said that employers already offering health benefits under collective bargaining agreements will be allowed to reduce how much they send to federal coffers — provided they pass those savings on to employees.

Democrats have spent the better part of three decades debating the proper role of government in health care, and the complicated politics surrounding the issue quickly resurfaced after Warren released her proposal. Biden, who favors building on the Affordable Care Act, slammed Warren’s plan as eliminating private insurance coverage and said it still amounts to a tax increase on workers.

President Donald Trump is sure to brand the effort as socialism.

For all the attention being paid to Warren’s proposal, Sanders is the chief architect of Medicare for All. He has previously released several options to pay for his measure, including a 4 percent income tax “premium” that kicks in after the first $29,000 for a family of four — very much affecting the middle class.