WASHINGTON — President Obama could leave office with the most aggressive, far-reaching environmental legacy of any occupant of the White House. Yet it is very possible that not a single major environmental law will have passed during his two terms in Washington.

Instead, Mr. Obama has turned to the vast reach of the Clean Air Act of 1970, which some legal experts call the most powerful environmental law in the world. Faced with a Congress that has shut down his attempts to push through an environmental agenda, Mr. Obama is using the authority of the act passed at the birth of the environmental movement to issue a series of landmark regulations on air pollution, from soot to smog, to mercury and planet-warming carbon dioxide.

The Supreme Court could still overturn much of Mr. Obama’s environmental legacy, although the justices so far have upheld the regulations in three significant cases. More challenges are expected, the most recent of which was taken up by the court on Tuesday. The act, however, was designed by lawmakers in a Democratic Congress to give the Environmental Protection Agency, which was created at the same time, great flexibility in its interpretation of the law.

“It’s the granddaddy of public health and environmental legislation,” said Paul Billings, a vice president of the American Lung Association. “It empowers the E.P.A. and states to be bold and creative.”