Donald Trump has been accused of "trying to bury" expert guidelines that said fully reopening schools and universities posed the highest risk of spreading coronavirus compared to partial or virtual openings.
American Federation of Teachers president Randi Weingarten said the president suppressed the report while pressuring the Centres for Disease Control and Prevention (CDC) to weaken safety recommendations in an effort to reopen schools faster.
"By trying to bury what the CDC recommends, Trump is betraying every student, teacher and parent in this country," Ms Weingarten, who leads the 1.7 million-member teachers union, said.
Ms Weingarten's tweet came after The New York Times revealed a 69-page CDC document that said "full-sized, in-person classes, activities and events" present the highest risk compared to entirely virtual or small classes with distancing.
"The more people a student or staff member interacts with, and the longer that interaction, the higher the risk of Covid-19 spread," the document, dated 8 July, said.
On the same day, Mr Trump said he disagreed with the CDC's "very tough & expensive guidelines" and threatened to cut off funding if schools did not reopen.
"The Dems think it would be bad for them politically if U.S. schools open before the November Election, but is important for the children & families. May cut off funding if not open!" he said.
Education Secretary Betsy DeVos was confronted by Fox News host Chris Wallace on Sunday over the president's threats to defund schools that did not reopen.
Asked "under what authority" the president could unilaterally cut off funding approved by Congress, Ms DeVos said if schools aren't going to fulfil the promise of an education, they shouldn't get the funds.
The Fox News host cut her off mid-sentence, saying "well, you can't do that".
"You can't do that unilaterally, you have to do that through Congress," Mr Wallace said.
Ms DeVos, who hasn't said whether schools should follow the CDC's guidelines, said they were "looking at all the options".
Last week, Vice President Mike Pence said during a press briefing on Wednesday that the CDC planned on releasing new guidelines this week that would not be as tough.
"Well, the President said today we just don't want the guidance to be too tough. And that's the reason why, next week, CDC is going to be issuing a new set of tools: five different documents that will be giving even more clarity on the guidance going forward," Mr Pence said.
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