Michelle Obama on Saturday aired her "genuine fear" that Donald Trump could retake the White House as the popular former first lady made a passionate appeal to voters in the desperately close US election.

Both Trump and Kamala Harris were in Michigan searching for holdout votes ahead of the November 5 election, with Harris focused on abortion rights and Trump returning to his anti-immigrant campaign theme.

Obama said Democratic candidate Harris would be an "extraordinary president of the United States" if elected in just 10 days.

But, with polls forecasting a virtual dead heat, she also spoke of a sense of frustration and anxiety that few on Harris's team dare express after she lost some momentum in recent weeks.

"My hope about Kamala is also accompanied by some genuine fear," Obama said, ripping into Trump's record and asking, "Why is this race even close?"

"I'm a little angry that we are indifferent to his erratic behavior, his obvious mental decline, his history as a convicted felon, a known slum lord, a predator found liable for sexual abuse."

Obama, appearing alongside Harris, hammered home the vice president's message that abortion rights -- and women's health care overall -- are at stake on the ballot.

"Please do not hand our fates over to the likes of Trump," Obama said, adding he could effectively ban abortion nationwide.

- Trump on the attack -

At his rally, Trump launched bitter personal attacks on Harris and accused her of pushing an "open border" migration policy.

"She's a dope," he said. "This person cannot be president."

"She will destroy our country. Everyone knows it. No one respects her. The United States is now occupied country. Kamala broke it, we will fix it."

With more than 38 million people already casting early ballots, Americans are deciding whether to elect the country's first-ever woman president, or its oldest commander in chief.

Trump, 78, still refuses to accept his defeat in the vote four years ago and is expected to reject the result if he loses again -- potentially pitching the United States into chaos.

Trump swept the three Blue Wall states -- Michigan, Wisconsin and Pennsylvania -- in his shock victory in 2016 only to see Joe Biden reclaim them four years later.

He hopes to claw back one or more of the trio, and win the so-called Sun Belt swing states to propel him back into power.

After his Michigan event, Trump headed straight to Pennsylvania for another rally on Saturday evening.

- Abortion rights -

Harris further highlighted abortion rights -- a weak point for Republicans -- by visiting a local doctor's office and meeting with physicians, staff and medical students.

"Because of Trump and what he did with the Supreme Court, we are looking at a health care crisis in America," she told reporters, referring to justices chosen by Trump who tipped the court into ending the national right to abortion in 2022.

The penultimate weekend before the vote began on Friday evening with Harris appearing alongside superstar Beyonce, and Trump giving a three-hour interview to Joe Rogan, America's most popular podcaster.

On Sunday Harris, 60, will campaign in Philadelphia, Pennsylvania, the largest city in the largest of the swing states likely to determine the election winner.

She will criss-cross the city, especially historically Black and Latino districts, trying to persuade uncommitted residents to cast their vote.

Trump will rally his supporters on Sunday evening in Madison Square Garden, the famous arena in the heart of heavily Democratic New York.

The brash billionaire and onetime reality television star appears keen to orchestrate a grand spectacle, and demonstrate he can fill an arena in a liberal bastion.

But critics, including Trump's 2016 rival Hillary Clinton, have noted that Madison Square Garden was also the scene of a 1939 pro-Nazi rally organized by a group supportive of Adolf Hitler.

Part of Harris's election strategy is to peel moderate Republicans away from Trump, who often demeans some Americans as the "enemy."

Harris told her supporters that as a prosecutor she "took on perpetrators of all kinds -- predators, fraudsters and repeat offenders. I took them on and I won.

"In 10 days, it's Donald Trump's turn."