Donald Trump used an expletive to attack Joe Biden during an address to an influential gun group Saturday, while the incumbent president warned in the crucial swing state of Georgia that his "unhinged" challenger is a threat to US democracy.

The dueling events came as Trump sought to make the most of his limited time on the campaign trail, with a criminal trial over hush money payments to a porn star keeping him mostly confined to a gloomy New York courtroom.

In a 90-minute rambling address to the politically powerful National Rifle Association, Trump said Biden is "the worst president in the history of our country by far" and suggested the Democrat was "full of bullshit."

"You're fired, get out of here Joe!" he called out to laughter from the thousands-strong crowd in Dallas, Texas, where the NRA gave the 77-year-old Republican its endorsement in the White House race.

Trump, who is fending off multiple criminal indictments for his unprecedented attempt to overthrow the results of the 2020 election and was twice impeached as president, told the crowd Biden is "crooked" and a "threat to democracy."

If Biden were Republican, he would be "given the electric chair," Trump said.

Biden meanwhile was stumping in the key southern state of Georgia as he sought support from African Americans -- a crucial element in his coalition.

The 81-year-old narrowly won Georgia in 2020 and some polls show Black voters are increasingly deserting him ahead of November's rematch with Trump.

"Our democracy is really on the line," Biden told supporters at Mary Mac's Tea Room, a Black-owned restaurant in Atlanta.

"My opponent's not a good loser. But he is a loser," Biden said to applause.

"He's clearly unhinged," Biden said, adding that something "snapped" in Trump after losing the 2020 election.

"Folks, Trump isn't running to lead America. He's running for revenge."

"We cannot let this man become president. Our children's future is at stake... We have to win this race, not for me but for America."

On Sunday Biden is due to speak at Morehouse College, a renowned historically Black university in Atlanta, then travel to Detroit where he will address the NAACP, the nation's top civil rights group.

- Trailing Trump -

A recent New York Times/Siena poll showed Biden trailing Trump in several key battleground states, despite Trump facing four criminal cases -- in Washington, Georgia and Florida in addition to New York.

Trump routinely claims the prosecutions are a conspiracy by Biden to prevent his return to the White House.

"Our enemies want to take away my freedom because I will never let them take away your freedom," he told the crowd in Dallas.

"In the end, they're not after me. They're after you. I just happened to be standing in their way."

Trump's appearance before the gun rights group comes after the White House last month moved to crack down on firearm sales at gun shows and online that evade federal background checks.

Biden has repeatedly called for a long-lapsed ban on military-style assault weapons to be reinstated, among other restrictions.

Warnings that Democrats would take away Americans' guns is a core theme at Trump's events, most recently at a fundraising dinner in Minnesota on Friday.

"If the Biden regime gets four more years they are coming for your guns, 100 percent certain," Trump said.

Efforts at broad reforms to gun laws have been stymied for decades, with executive actions and state initiatives attacked in court by critics as infringing on the constitutional right to own a firearm, enshrined in the Second Amendment.

The 150-year-old NRA, whose longtime CEO resigned in January amid a graft lawsuit, is closely aligned with the Republican Party in opposing firearm restrictions, despite routine mass killings in the United States and high rates of gun violence compared to peer nations.

There were more than 40,000 gun-related deaths in the United States last year, according to the Gun Violence Archive.