In Europe, the weedkiller atrazine has been banned for nearly two decades because of its suspected links to reproductive problems like reduced sperm quality and birth defects.

In the United States, it remains one of the most widely used pesticides, sprayed on corn, sugar cane and other crops, the result of years of industry lobbying. It has been detected in the drinking water of some 40 million Americans.

Now, American environmental groups that have long sought a ban are finding some unexpected allies: the Trump administration and its MAGA supporter base.

This week, a “Make America Healthy Again” commission led by Health and Human Services Secretary Robert F. Kennedy Jr. is set to issue a report on the causes of chronic illnesses in the United States. And Mr. Kennedy, who worked for years as an environmental lawyer fighting chemical companies, wants the report to highlight the harms of pesticides like atrazine, according to three people with knowledge of his efforts.

It is an unwieldy coalition, extending even to some men’s rights influencers on alternative media, where commentary abounds on how toxic chemicals are threatening masculinity.

They are taking on an influential agricultural and chemicals lobby that has long rebuffed attempts to strengthen restrictions on atrazine and other pesticides, at a time when the Trump administration is rolling back government restrictions on industries, not imposing new ones.