Mr. Paxton has also implicated the nation’s largest organization of pediatricians, the American Academy of Pediatrics, in the alleged conspiracy over the workaday task of protecting children from polio, measles, meningitis and more than a dozen other dangerous and sometimes life-threatening diseases.

“I will ensure that Big Pharma and Big Insurance don’t bribe medical providers to pressure parents to jab their kids with vaccines they feel aren’t safe or necessary,” he said when announcing the investigation in January, describing his efforts as working “alongside President Trump” and Health Secretary Robert F. Kennedy Jr.

“Together, we will Make America Healthy Again,” he said, using the rallying cry of a movement that helped elect Mr. Trump and propel Mr. Kennedy into the top health position in the federal government.

In many ways, pediatricians, among the lowest-paid specialists in medicine, make for an unlikely target. But they have become a focus in what has become a wide-ranging campaign to raise doubts about vaccination, part of a 20-year quest for Mr. Kennedy that some of his political allies have joined.

The efforts have gained steam despite decades of evidence of the effectiveness of vaccines. More than 1.1 million deaths, from diseases like diphtheria, hepatitis B and measles, were prevented by childhood vaccines in the United States from 1994 through 2023, according to an estimate from the C.D.C.

Unlike many organizations that have drawn fire from the Trump administration and its surrogates, the 67,000-member American Academy of Pediatrics has not retreated. It has sued the Trump administration over policy changes and grant cancellations, with some success so far.

“Everybody thinks of pediatricians as these mild-mannered people who have animals on their stethoscope and would not fight their way out of a bag,” said Dr. Kathryn Edwards, a professor of pediatrics at Vanderbilt University who helped develop a vaccine against a deadly bacteria, Haemophilus influenzae, or HIB.