In an interview on Sunday, Mr. Sanders described how seeing the Canadian system up close significantly shaped his own views on health care.

“It was kind of mind blowing to realize that the country 50 miles away from where I live — that people could go to the doctor whenever they wanted and not have to take out their wallet,” he said.

“That was just a profound lesson that I learned,” he said.

He also criticized the American system as “barbaric.” And he vowed — as he often does in his stump speeches — “to take on the greed and the corruption of the health care industry.”

Mr. Sanders’s health care proposal has attracted legions of supporters fed up with the rising costs of the current system, and it sets him apart from more centrist candidates like Joseph R. Biden Jr. But his uncompromising position also threatens to alienate voters who are pleased with the Affordable Care Act, or who do not want to give up their private insurance. His own state of Vermont so far does not have a single-payer program.

Despite skepticism about his views, however, Mr. Sanders has consistently resolved to reform the health care system, even before being elected to public office. In 1972, when he was running for Senate as a candidate from Vermont’s left-wing Liberty Union Party, The Bennington Banner, a local newspaper , reported him taking an uncompromising stance: “There is absolutely no rational reason, in the United States of America today, we could not have full and total free medical care for all.”

The challenge of paying medical bills

The first seeds of Mr. Sanders’s concern were sown in Brooklyn.

A high-school track and cross country star with an emerging political streak, Mr. Sanders had wanted to go to Harvard, friends said. But by his senior year, his mother, Dorothy Sanders, had become sick, her heart damaged from having rheumatic fever as a child.