Teachers nationwide are being targeted in a campaign to spread bogus information about climate change, four US senators warned the secretary of education in a letter sent on Wednesday and shared with BuzzFeed News.

Packages holding a cover letter, a 135-page book, and an 11-minute DVD, all falsely claiming that there is no scientific consensus on man-made climate change, started arriving in teacher mailboxes in March. The mailings were sent to more than 300,000 teachers, according to the group behind the campaign, the Heartland Institute. Based outside of Chicago, Heartland is a conservative think tank that lobbies against climate regulation, has received funding from fossil fuel-linked groups, and lauded President Donald Trump’s recent decision to withdraw from the Paris climate agreement.

“The Heartland Institute has disseminated 'alternative facts' and fake science at the behest of its industry funders for decades,” reads the letter to Education Secretary Betsy DeVos, signed by Democratic senators Sheldon Whitehouse, Edward Markey, Elizabeth Warren, and Brian Schatz.

Heartland does not dispute that the purpose of the mailers, which included the organization’s book, Why Scientists Disagree About Global Warming, was to encourage teachers to challenge mainstream climate science.

“The intent was to make sure teachers were aware that there was alternative scientific research,” Heartland spokesperson Jim Lakely told BuzzFeed News.

“There’s been a positive response to the mailings for sure,” Lakely said. Responding to requests from schools, Heartland staff visited some classrooms this spring, he added. “We have some invitations pending for the next school year when it begins.”

Earlier this week, Whitehouse, an outspoken advocate for action on climate change, sent a similar letter to a dozen science education and teacher associations, debunking the Heartland information. For some teachers, the mailers were already a source of frustration.

“I’m appalled at the Heartland Institute’s gall to think we are dumb enough to buy into this,” said Cheryl Manning, a science teacher at Evergreen High School in Colorado who, along with the school’s seven other full-time science teachers, received Heartland’s package last week.

Manning thought that the materials looked professional and scientific. “It’s just loaded with citations,” she said. “But it’s circular. It’s all self-citations. Citing their own stuff instead of citing other people’s work.”

Besides teaching earth science to ninth graders and advanced chemistry to twelfth graders, Manning serves as the president of the National Earth Science Teachers Association, a group of about 1,000 teachers in the US. She was already drafting a statement to members about the Heartland mailers when she heard from Whitehouse’s office about the campaign.

“I was very happy to see there was this kind of support in the legislature,” Manning said.

Whitehouse’s letter not only aims to debunk the contents of the Heartland Institute’s mailer — summarizing the latest climate science and offering legitimate sources for teachers such as the government-run website Climate.gov — but also asks teacher organizations to fight back.

“I hope you will work to address industry-driven efforts to affect what is being taught in the classroom,” Whitehouse wrote.

At least one of the science organizations that Whitehouse wrote to has already done this. The National Science Teachers Association sent a letter to its members in April about the “misleading teaching information from a questionable source” and posted recommended climate resources for members on its website.

In their letter to DeVos, Whitehouse and his colleagues questioned her stance on climate change and asked whether any Department of Education staff have been in contact with Heartland-linked policy experts.

When asked about climate change during her confirmation process, DeVos responded: “The Department of Education is prohibited from dictating curricula in our nation’s schools. If confirmed, I would respectfully defer to my colleagues in other agencies, like the Department of Energy, on the issue of climate change.”

DeVos didn’t say much more about climate until last week. Responding to Trump’s announcement to pull the US from the Paris accord, DeVos said in a statement that the decision “is one more example of his commitment to rolling back the unrealistic and overreaching regulatory actions by the previous Administration.” When DeVos was asked about her personal views on climate change, she said: "Oh I think that certainly the climate changes. Yes."