Trump toured the Ford plant, which has been recast to produce ventilators and personal protective equipment, and held a roundtable discussion with African-American leaders concerning vulnerable populations hit by the virus.

The president, who has said he is taking a drug not proven for the coronavirus after two White House staffers tested positive in recent weeks, did not wear a mask during any of his public events at the plant in the city of Ypsilanti even though Ford on Tuesday reiterated its policy that all visitors must wear them.

Trump has consistently disregarded guidance from the US Centers for Disease Control and Prevention urging people to wear masks in close company to try to curb the spread of the virus.

Surrounded by Ford executives who were wearing masks, Trump told reporters he had put one on out of the view of cameras,

"I had one on before. I wore one in the back area. I didn't want to give the press the pleasure of seeing it," Trump said.

When asked if Trump was told it was acceptable not to wear a mask in the plant, Ford Executive Chairman Bill Ford said, "It's up to him."

"Honestly I think I look better in a mask," Trump added jokingly.

Trump, a Republican seeking re-election on November 3, has urged states to loosen coronavirus-related restrictions so the battered US economy can recover even as public health experts warn that premature relaxation of restrictions could lead to a second wave of infections.

Michigan Governor Gretchen Whitmer, seen as a potential vice presidential running mate for presumptive Democratic presidential nominee Joe Biden, is facing a backlash from some critics against her stay-at-home orders.

Trump has encouraged anti-lockdown protests against Whitmer held in Michigan's capital.

Whitmeron Thursday moved to further reopen Michigan's economy through a series of executive orders.

Trump on Wednesday threatened to withhold federal funding from Michigan over its plan for expanded mail-in voting, saying without offering evidence that the practice could lead to voter fraud - though he later appeared to back off the threat.

At the plant, Trump reiterated his opposition to mail-in voting, saying, "Voting by mail is wrought with fraud and abuse." Democrats have accused Republicans of pursuing voter suppression policies aimed at voters who tend to back Democrats.

Whitmer spoke with Trump on Wednesday.

"I made the case that, you know, we all have to be on the same page here. We've got to stop demonizing one another and really focus on the fact that the common enemy is the virus," Whitmer said after the meeting.

Australian Associated Press