About 27 percent of people under 65 are thought to have some sort of pre-existing condition that will most likely leave them without individual insurance if the law is repealed, according to a recent study. The guarantee of coverage has already become a rallying cry for people who want to keep the law.

The issue “is the third rail” for the Republicans, said Michael Turpin, a longtime health industry executive.

Before the law, a fairly typical life event — like a divorce or the loss of a job — and a relatively minor medical condition could upend a person’s health coverage options. Stories of sick people unable to get coverage when they needed it most were legion.

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