From the moment the Republican-controlled House of Representatives approved a plan to overhaul the health care system, an onslaught of opposition to the bill has been focused on a single, compact term: pre-existing conditions.

The Democratic Congressional Campaign Committee began running digital ads warning that the legislation would leave “no more protections” for people with a history of illness or injury. Pointing to the power that states could have to set the terms for insurers under the G.O.P. bill, Democratic leaders announced they would make pre-existing conditions an issue in every gubernatorial and state legislative race in the country.

Groups on the left posted graphics online listing pre-existing conditions that could, in theory, threaten health care coverage, with some shared hundreds of thousands or millions of times. In one exaggerated claim circulating widely on social media on Friday, a post from a group called The People for Bernie Sanders listed nearly 100 conditions, from AIDS to ulcers, and said that for anyone who suffered from them, Republicans had voted “to end your health care.”

Individuals took up the call, too: More than 100,000 people posted on Twitter using the hashtag #IAmAPreexistingCondition, with many naming their own long-term illnesses or medical conditions.