AMNA NAWAZ: When he left the presidential race, Robert F. Kennedy Jr.

galvanized his supporters to vote for Donald Trump, which helped to propel Trump back to the White House.

The president promised to let Kennedy go wild on health care policy.

But as fans of Kennedy's Make America Healthy Again, or MAHA, movement have learned, that promise has limits.

And now some MAHA voters are feeling disillusioned.

Ali Rogin recently spoke to some of them.

NANCY FULLER, MAHA Supporter: I was on a bridge during rush hour.

ALI ROGIN: Ohio mom Nancy Fuller was waving the flag on the nation's health problems long before she supported Robert F. Kennedy Jr.

for president.

Her motivation?

Her son Steve, now 33, a visual artist with autism and epilepsy.

NANCY FULLER: Steve's art is so unique.

And what's fascinating for most artists to appreciate is that there is no draft.

ALI ROGIN: As a child, Steve was nonverbal, and doctors warned he would have a lower quality of life.

Fuller says they had few answers, so she sought them out herself.

I know that your journey with Steve has made you MAHA before MAHA was a slogan.

NANCY FULLER: Exactly.

Steve is like a canary in the coal mine.

He's very sensitive to toxicities in our air, food and water.

And he's singing, and he's calling out to be helped.

ROBERT F. KENNEDY JR., U.S.

Health and Human Services Secretary: Most American children can no longer go fishing with their father and come home and safely eat the fish.

ALI ROGIN: Fuller began following Kennedy as his profile rose as an environmental lawyer and activist speaking out about clean water and pesticides, including against agrochemical company Monsanto, creator of the herbicide Roundup.

NARRATOR: Kills the root, kills the weed.

ALI ROGIN: In 2018, Kennedy helped win an almost $300 million settlement for a man who blamed his cancer on Roundup's active ingredient, glyphosate.

The World Health Organization labeled glyphosate probably carcinogenic to humans in 2015.

Kennedy's controversial opinions on vaccines also resonated with Fuller, who describes herself as -- quote -- "vaccine-hesitant."

She questions whether vaccines played some role in Steve's autism.

Dozens of studies on different vaccines have found no connection to autism.

NANCY FULLER: I am a proponent, actually, of vaccines, but I also think there's a subpopulation of people that are very sensitive.

There could be a contributing factor of the vaccines and the environment to what kind of set off the autism in my son.

I do believe that.

ALI ROGIN: Fuller was thrilled when Kennedy ran for president, but the lifelong Democrat was torn when he dropped out and endorsed Trump.

NANCY FULLER: I did have to kind of hold my nose, because my vote for that ticket was a vote for RFK Jr.

ALI ROGIN: And a vote for him to make good on his campaign promises.

ROBERT F. KENNEDY JR.

: We're going to ban the worst agricultural chemicals that are already prohibited in other countries.

ALI ROGIN: But, in February, the president issued an executive order to increase domestic production of glyphosate, citing national security concerns.

Then Kennedy gave Trump political cover, writing on social media: "I support President Trump's executive order to bring agricultural chemical production back to the United States and that Trump did not build our current system.

He inherited it."

Like many MAHA supporters, Fuller was furious.

NANCY FULLER: When RFK Jr.

put out his statement on glyphosate, I would hope that I would have been a little more honest about it.

I do believe he knows the harm this toxic chemical does to people.

ALI ROGIN: And now the drug company Bayer which owns Monsanto, is lobbying state and federal lawmakers to pass laws protecting it from health-related lawsuits.

WOMAN: Thank you for standing up for our next generation.

ALI ROGIN: Last month, the Supreme Court heard its case as MAHA supporters rallied outside.

The Trump administration urged the court to rule in favor of Bayer.

JON MARTIN, Rallygoer: Well, I will tell you, I'm a conservative and I do support President Trump, but I do not agree with everything he does.

And this is one thing that I was highly disappointed that he did this.

SHERRI BEHNEY, Rallygoer: I thought he would be on board with especially holding people accountable for things.

And having RFK Jr.

in there too I thought would help.

So it is very disappointing.

ALI ROGIN: A recent Politico poll found that a third of Americans now identify as MAHA supporters, and half of them view limiting pesticide use as a core issue of the movement.

ALEX CLARK, Host, "Culture Apothecary": I knew that, if there was an issue with moms that could make or break the MAHA coalition, it was pesticides.

ALI ROGIN: Alex Clark is the host of "Culture Apothecary," a podcast produced by the young conservative group Turning Point USA.

She said her listeners, many so-called MAHA moms, felt betrayed by Trump's executive order on glyphosate.

ALEX CLARK: You know, these voters want fight, fight, fight Trump.

They don't want ballroom Trump.

They want the man who promised to fight the system.

And for MAHA moms, that includes big ag and big chemical.

ALI ROGIN: In what was viewed as a White House attempt to make amends with MAHA, Clark was part of a delegation who met with Trump and other officials last month.

ALEX CLARK: We laid out how upset we were and how upset MAHA voters are on the pesticide issue.

MAHA could absolutely be decisive in the midterms.

The political power of MAHA is that these voters are movable.

And in a midterm, movable voters decide everything.

ELIZABETH FROST, MAHA Ohio: Early on, we were just super, super grassroots.

ALI ROGIN: Voters like Elizabeth Frost, a political consultant and activist in Southern Ohio who worked on Kennedy's presidential campaign.

ELIZABETH FROST: It was such an interesting experience to see this massive tent that Kennedy was able to cast.

People showed up to knock doors for Trump, people who could barely stomach the thought of it, because that was what they wanted to do for the sake of advancing MAHA policies.

ALI ROGIN: Frost now leads the grassroots group MAHA Ohio.

Like many Kennedy supporters who adopted the MAHA label, she feels the Trump administration is dismissing this valuable group.

ELIZABETH FROST: You know, having Kennedy there beside you for your press conferences is not enough.

If Kennedy as the head of HHS isn't being fairly listened to on this issue, we have got a number of major problems for this relationship.

ALI ROGIN: Frost says, ahead of the midterms, she's interested in candidates who aren't afraid to challenge Trump on issues like pesticides, like Republican Thomas Massie in neighboring Kentucky.

He co-wrote a bipartisan bill banning glyphosate manufacturers from legal immunity, but he lost to a Trump-endorsed candidate in his primary.

Would you prefer a candidate backed by Trump and MAGA or someone backed by Kennedy and MAHA?

ELIZABETH FROST: We're very much more aligned with MAHA values than anything that we're seeing coming out of the White House currently, and people are going to show up to vote for change.

ALI ROGIN: Back in Cincinnati, Nancy Fuller says her MAHA values will be a filter in the midterms and that she isn't ready to give up on Kennedy.

NANCY FULLER: I'm not in his shoes, and he's got to pick his battles to bring health to America.

And if that's something that the president of the United States wants to do, I don't want to see him get fired.

I think at the end of the day it's a chess game for him, and he has to move the chess pieces where he can advance health.

ALI ROGIN: In the game of electoral politics, MAHA voters can be kingmakers, and they still have plenty of moves left to make.

For the "PBS News Hour," I'm Ali Rogin in Cincinnati, Ohio.