Donald Trump ignited a new feud with the attorney general of Michigan after she called him a "petulant child" for refusing to wear a face mask in public on a visit to the state.
The US president declined to wear a mask in front of cameras at a Ford plant, which has been making ventilators, although he did put one on for a private part of the tour.
Dana Nessel, the Michigan attorney general, also took aim at Ford, threatening legal action against the company for allowing Trump to violate the state governor's executive order on masks. The order stipulates that any person who is medically able must wear one in enclosed public spaces.
Trump responded on Twitter: "The Wacky Do Nothing Attorney General of Michigan, Dana Nessel, is viciously threatening Ford Motor Company for the fact that I inspected a Ventilator plant without a mask.
"Not their fault, & I did put on a mask. No wonder many auto companies left Michigan, until I came along!"
He added that Nessel "should not be taking her anger and stupidity out on Ford Motor".
Nessel, a Democrat, said Trump had a "legal and moral responsibility" to wear a mask, and had sent a "terrible message" by not doing so.
She said: "The president is like a petulant child who refuses to follow the rules. This is no joke. He is a ridiculous person, and I am ashamed to have him be president of the United States of America."
Michigan, hit badly by floods in recent days, will be a key state in the November 3 presidential election.
Trump won it narrowly, and unexpectedly, over Hillary Clinton in 2016.
Gretchen Whitmer, Michigan's Democrat governor, is seen as a potential vice-presidential running mate for Joe Biden, the presumptive Democrat nominee.