WASHINGTON  It was just one line in a campaign manifesto, and it hardly seemed the most significant or contentious. As a presidential candidate, Barack Obama said he would “establish a new public insurance program” alongside private health care plans.

That proposal took on a life of its own, but it now appears to be dying, a victim of an ineffectual White House strategy, the president’s failure to argue passionately for the “public option” and all-out opposition by the insurance industry and much of the health care industry.

In the campaign, Mr. Obama said the public plan would compete with private insurers on the price and quality of care, thus benefiting consumers. What Mr. Obama did not foresee is that, to some people on the right and the left, it would become the most important issue in the debate over health care, touching off a battle over the role of government in one of the nation’s biggest, fastest-growing industries.

Once in office Mr. Obama and his advisers have sent conflicting signals about how critical a government-run health plan would be. He prefers a public plan but is open to other ideas.