Presidential candidate U.S. Sen. Bernie Sanders (I-Vt.) has made health care a central plank in his platform. At a rally on July 30 to commemorate the 50th anniversary of Medicare, Sanders declared that "we need to expand Medicare to cover every man, woman and child as a single-payer national health-care program."

Sanders also pledged that he would introduce legislation in the near future that would enshrine health care as a right.

Sanders is unique among presidential candidates in his support for a single-payer system – one that goes further than the Affordable Care Act (ACA) in providing not only universal coverage, but also easier access to affordable health care.

A single-payer system is one in which a single entity pays for medical bills, but there is variation in how governments around the world have implemented this system. In Canada, for instance, each province provides a health insurance plan to all residents, but health care providers operate privately. In the United Kingdom and Scandinavian countries like Norway and Sweden, the government owns the providers and pays them.

Such a system has moderate support from Americans: polls put approval of a single-payer system between 37 and just over 50 percent.

Sanders, along with Sen. Ron Wyden (D-Ore.), successfully advocated for the inclusion of "innovation waivers" in the ACA. According to Section 1332 of the health care law, beginning in 2017, states can apply for waivers that allow them to use federal subsidies for reforms that allow them to carry out the purpose of the ACA – quality and affordable care – with considerable flexibility.

Hawaii, for instance, established a task force in 2014 to determine how the state could best utilize the waivers.

In 2009, Hawaii's legislature passed a law to establish a board that would build a universal coverage plan, and then overrode the governor's veto. Subsequent governors have expressed support for a single-payer system, but the establishment of the state's health care exchange – which has been especially costly, complicated, and unpopular – has stalled the movement for single-payer.

Since 1976, Hawaii has mandated that companies offer insurance to employees who work more than 20 hours per week.

California is also looking to apply for a waiver, which would allow all Californians, including undocumented immigrants, to purchase insurance through Covered California, the state's exchange.

California has nearly adopted a single-payer system several times over the last decade. In 2006 and 2008, the legislature passed a single-payer bill, but it was vetoed both times by then-Governor Arnold Schwarzenegger. In 2010, another bill sponsored by State Senator Mark Leno passed the upper chamber, but did not clear the lower house. In 2012, the reverse occurred: the bill passed in the Assembly but failed in the Senate.

Perhaps no state has come closer to adopting a single-payer system than Vermont.

In 2011, Vermont's legislature passed Act 48, which put the state on the path to developing a single-payer system. However, in December 2014, Gov. Peter Shumlin abandoned the plan. To finance the system, employers would have had to pay 11.5 percent in payroll taxes. However, businesses that operated in multiple states would have been exempt, causing residents to have to contribute up to 9.5 percent of their salaries toward their premiums.

planned to offer the most expensive of plans to its residents – platinum, which are considerably more expensive than the silver plans used as "the benchmark" for subsidies under the ACA.

William Hsiao, a Harvard economist who helped design Vermont's system, estimated that the plan would have reduced the state's health care spending by as much as 25 percent and thus offset the higher taxes.

According to Linda Bergthold, a health policy consultant who blogs at Huffington Post, at least 22 states around the country are part of "an organized national single-payer network, ready to utilize the provision in the ACA allowing for innovation as of January 1st 2017." Other states in this network include Pennsylvania and New York, where activist groups and lawmakers have advocated for single-payer reform.

In May, the state Assembly in New York passed the "New York Health Act," a bill sponsored by Richard Gottfried, who has been advocating for single-payer for 20 years. The proposed system would have reduced health care spending by an estimated $45 billion by 2019, largely as a result of eliminating the administrative costs associated with for-profit insurance companies.

The bill was not passed by the Republican-controlled Senate.

Since Sen. Joe Lieberman opposed the inclusion of a public option in the Affordable Care Act in 2009 when the Democrats possessed supermajorities in both houses, there is little evidence that Bernie Sanders would be able to convince any Congress – let alone a Republican one – to expand Medicare to all if he is elected president.

However, his success in including Section 1332 in the ACA may enable states to adopt the single-payer system he longs to see implemented – one state at a time.