Sen. Bernie Sanders is drawing unprecedented support from Democrats for his proposal to implement a government-run healthcare system in the U.S., but the bill doesn't address several questions that are likely to emerge as some lawmakers advocate on behalf the legislation.

Under the Sanders healthcare plan, called the Medicare for All Act, Medicare would pay for emergency surgery, prescription drugs, mental healthcare and eye care without a co-pay. Soon after it would be passed into law, people 18 and under would receive a "universal Medicare card" and others currently not eligible for Medicare, even those who now have private plans, would be phased into the program after four years.

Here are some of the questions that the bill does not answer or address in detail:

1. How will the U.S. pay for it?

One of the first criticisms to emerge about the Sanders bill, from Republicans and from the media, is how it would be financed. It's a question that has dogged advocates of a single-payer healthcare system in the U.S. for a long time, and one that individual states have been unable to address. State lawmakers in Sanders' home state of Vermont, for instance, abandoned plans to implement a single-payer healthcare system after they discovered how costly it would be.

The bill has not been scored by the Congressional Budget Office, but an analysis of a similar plan Sanders introduced when he was running for the Democratic nomination for president, from the left-leaning Urban Institute, projected that such a system would increase federal spending by $32 trillion over a decade. With more costs shifting to the federal government, the analysis projected that the private sector would spend $22 trillion less than it otherwise would have and states would spend $4 trillion less.

Sanders has said that he and other senators who support the proposal will be traveling across the country to tell Americans about the healthcare plan and to receive input from the public and from members of the hospital industry about how to pay for it.

Several notes in an explainer distributed by Sanders' office show different ways that $16 trillion could be raised, including through taxes on employers and households, but it isn't clear that all these provisions would be employed.

2. How will the bill deal with the provider shortage?

The bill does not address ways that it would fund training and hiring of more healthcare professionals.

People who live in countries with a single-payer healthcare system have complained about long waiting times for a doctor's appointment or procedure, and the U.S. already faces shortages in providers. Long wait times can lead to worsening conditions, and shortages can contribute to physician burnout.

By 2025, the U.S. will face a shortage of between 46,100 and 90,400 doctors, according to the Association of American Medical Colleges, and the shortage in primary care alone is between 12,500 and 31,100 doctors.

The Medicare for All Act also would cover mental health. The U.S. has one mental health professional, whether a psychiatrist, psychologist or other behavioral health provider, per 1,000 people in states with the lowest workforce, according to Mental Health America.

Sanders blasted health insurance and drug companies for their profits as he was introducing his single-payer plan. Under the current U.S. healthcare system, however, most spending on healthcare goes to doctors and hospitals, who would not only face a higher influx of patients under a single-payer system, but likely also would see reductions in their bottom lines, given that Medicare reimbursements are less than private insurance. That would spell trouble for the industry in the form of lower salaries for doctors, laid-off staff and hospital shutdowns.

3. How will it pay for medical care for illegal immigrants?

Under single payer, when a person comes to the U.S. illegally and accesses medical care, or is rushed to an emergency room, would the government pay for it?

Sanders' bill stipulates that someone must be a U.S. citizen or have immigrated lawfully to qualify for Medicare, but under the current healthcare system, people who are in the country illegally have other means to receive coverage.

While under Obamacare they do not qualify for federally subsidized coverage for to help pay for premiums, or for Medicare and Medicaid, under the Emergency Medical Treatment and Active Labor Act, hospitals are reimbursed when they provide emergency and maternity care for low-income people who need it, regardless of immigration status. Immigrants in the country illegally also can receive medical care through community health clinics or buy coverage directly from an insurer. Such private comprehensive plans, however, would be largely phased out.

4. What constitutes healthcare?

While the bill mentions that people could buy private coverage for voluntary procedures, such as plastic surgery, it doesn't get into detail about some of the other types of medical care involving social issues that policymakers were debating during the last few years of former President Barack Obama's term. The bill would, however, pay for abortions.

Taxpayers may object to this, or to paying for end-of-life care that includes allowing people with a terminal illnesss to take life-ending medications, even though it is paid for by Medicaid in states where it is legal.

And several topics that have been at the forefront of discussion, particularly among Democrats, are not mentioned in the bill. For instance, the Obama administration supported the LGBTQ community by removing a ban against Medicare funding for gender-reassignment surgery and it endorsed same-sex marriage. The Medicare for All Act doesn't say whether gender-reassignment procedures would be covered, and it does not address the financial barriers same-sex couples face in accessing assisted reproductive technologies, which can include IVF and a surrogate.

There is also a movement within the medical profession to broaden the definition of healthcare to include prescriptions for healthy food. And federal dollars on public health funding have gone toward exercise programs, which some advocates say are an important part of prevention.

The bill indicates the Department of Health and Human Services will routinely consider which services receive coverage, but such an arrangement means that certain provisions in Medicare would be contingent on the politics of the person holding the Oval Office.