Joe Biden had just rolled out his health care plan when he made what could be a fateful pledge to a crowd in Iowa: “If you like your health care plan or your employer-based plan, you can keep it.”

Biden’s promise on job-based coverage, which almost 160 million Americans use, underscores the risks of positioning himself as the health overhaul’s chief defender. Fully embracing the health law and pledging to expand it also means exposing Biden to attacks from all sides: from the left that wants more than what Biden is offering; from the right that loathes the law in any form; and from the middle, where voters remain skeptical about the nation’s complex and expensive health care system. […]