WASHINGTON (AP) — A day after threatening to withhold funds over Michigan’s mail-in ballot effort, President Donald Trump was set to visit the electoral battleground Thursday to learn how the medical breathing machines governors begged for during the height of the coronavirus pandemic are made.

But hovering over his visit to a repurposed Ford Motor Co. factory in Ypsilanti, outside of Detroit, is whether he will wear a face covering.

Trump has refused to wear a face mask in public, a practice that federal health authorities say all Americans should adopt to help slow the spread of the virus.

Ford says everyone in its factories must wear personal protective equipment, including masks, and that its policy has been communicated to the White House. Trump wouldn’t commit to wearing a mask at the plant when he was asked about the matter earlier this week.

Michigan Attorney General Dana Nessel said that mask-wearing isn’t just Ford’s policy but it’s also the law in a state that’s among those hardest hit by the virus. She sent Trump a letter Wednesday asking him to respect Ford’s employees by covering his nose and mouth during the visit, which will include a roundtable discussion with black leaders, a factory tour and speech.

“Anyone who has potentially been recently exposed, including the president of the United States, has not only a legal responsibility, but also a social and moral responsibility, to take reasonable precautions to prevent further spread of the virus,” Nessel said.

At least two people who work in the White House and had been physically close to Trump recently tested positive for the virus. Trump is tested daily.

On Wednesday, Trump threatened to withhold federal funds from Michigan after its secretary of state mailed absentee ballot applications to millions of voters. Trump first tweeted — erroneously — that the Democratic state official had mailed absentee ballots to Michigan voters. He later sent a corrected tweet specifying that applications to request absentee ballots had been mailed.

Trump narrowly won Michigan in 2016. He insists mail-in voting is ripe for fraud, although there is scant evidence of wrongdoing.