WASHINGTON — Back in 2009, when Rand Paul was pursuing his long-shot bid to win Kentucky’s Republican Senate primary, he spoke to a small physicians’ association that has publicized discredited medical theories, including possible links between vaccines and autism and between abortion and an increased risk of breast cancer.

At the time, Mr. Paul, an ophthalmologist, was no stranger to the group, the Association of American Physicians and Surgeons. He boasted at its annual meeting that he had been a member for more than two decades and that he relied on its research, statistics and views about the role of government in medicine.

“I am not a newcomer to AAPS,” Mr. Paul said, referring to the group.

On Monday, Mr. Paul helped set off an uproar when he said amid a national measles outbreak that parents should be allowed to decide whether their children needed to be vaccinated, and that he had heard from parents whose children had suffered “profound mental disorders” after being vaccinated.

In doing so, he was echoing the views of the head of the association, which has also lobbied in recent years for state laws permitting parents to opt out of mandatory inoculation programs based on their beliefs.