The Supreme Court ruled on Thursday that President Obama’s health care law allows the government to provide nationwide tax subsidies to help poor and middle-class people buy health insurance, a sweeping vindication that endorsed the larger purpose of Mr. Obama’s signature legislative achievement.

The 6-to-3 ruling means that it is all but certain that the Affordable Care Act will survive after Mr. Obama leaves office in 2017, and will give it a greater chance of becoming an enduring part of America’s social safety net.

For the second time in three years, the law survived an encounter with the Supreme Court. But the court’s tone was different this time. The first decision, in 2012, was fractured and grudging, while Thursday’s ruling was more assertive. NYT