BURLINGTON, Vt. — Just a few years ago, lawmakers in this left-leaning state viewed President Obama’s Affordable Care Act as little more than a pit stop on the road to a far more ambitious goal: single-payer, universal health care for all residents.

Then things unraveled. The online insurance marketplace that Vermont built to enroll people in private coverage under the law had extensive technical failures. The problems soured public and legislative enthusiasm for sweeping health care changes just as Gov. Peter Shumlin needed to build support for his complex single-payer plan. Finally, Mr. Shumlin, a Democrat, shelved the plan in December, citing the high cost to taxpayers. He called the decision “the greatest disappointment of my political life.”

As the United States Supreme Court prepares to rule in a case that could gut a major element of the Affordable Care Act — federal subsidies for low- and middle-income people — Vermont should have little to worry about. Only states that use the federally run insurance marketplace stand to lose subsidies if the court rules against the Obama administration, and Vermont is among the 14 states that fully run their own.

But even though its residents’ subsidies appear safe for now, Vermont stands as a cautionary tale. Despite an eventual cost of up to $200 million in federal funds, its online marketplace, or exchange, is still not fully functional, while disgust with the system is running deep among residents and lawmakers alike.