Indianapolis-based Anthem included a similar point in a full-page ad it put in many national newspapers in July.

Sanders said in a statement that it is disappointing to see that Aetna has joined other large for-profit health insurance companies in pulling out of the ACA exchange system. "

"Despite the Affordable Care Act bringing them millions more paying customers than ever before, these companies are more concerned with making huge profits than ensuring access to health care for all Americans," Sanders said in the statement.

"In my view, the provision of health care cannot continue to be dependent upon the whims and market projections of large private insurance companies whose only goal is to make as much profit as possible," Sanders said. "That is why we need to join every other major country on earth and guarantee health care to all as a right, not a privilege. That is also why we need to pass a Medicare-for-all single-payer system. I will reintroduce legislation to do that in the next session of Congress, hopefully as part of the Democratic Senate majority."

The next Congress is set to start in January 2017.

Sanders' past American Health Security Act bills would let health care providers collect reimbursement only from state-run health plans, not from patients or private insurers. (Image: Thinkstock)

How Sanders' single-payer plan might work

Advocates of a "public option" program want the government to run a public health insurance program for working-age people that operates alongside commercial health insurance plans.

In a pure single-payer system, a government health insurance program acts as the sole source of health coverage.

Sanders argued in 2010 that offering universal access to a public coverage option would be better than the system that was in place in 2010. But, when Sanders has introduced his own American Health Security Act bills, he has supported moving the country to a pure single-payer system.

In Sanders' American Health Security Act bills, he has proposed that each state should have its own publicly funded health plan and health plan board.

Each state's plan would pay for all medically necessary acute medical care and long-term care.

A state's health plan board would negotiate reimbursement rates with the state's health care providers.

Providers could collect payments only from the state plans, not from the patients or private insurers.

If a state's health plan started to run out of cash, the plan would balance its books by cutting its provider reimbursement rate until the start of the next fiscal year.