President Biden declared on Saturday that his challenger, former President Donald J. Trump, represented an “unhinged” threat to the future of the country and asked Black voters at two campaign events in Atlanta to see the election as a choice between protecting democracy and letting it backslide.

This message was a preview of sorts for a speech he was scheduled to deliver on Sunday at Morehouse College, an all-male, historically Black institution whose students, alumni and faculty had been divided over inviting Mr. Biden as the war in Gaza continues.

Mr. Biden laid out his argument to a powerful slice of the electorate that has been drifting away from him during a campaign reception on Saturday afternoon: “We cannot let this man become president. We have to win this race, not for me but for America.”

For months, the president has tried to define Mr. Trump as an unstable force whose second term would be about exacting revenge on his enemies. But despite trying to present himself as a guardian of the international order and politics as usual, Mr. Biden has low approval ratings and is trailing Mr. Trump in several battleground states including Georgia, according to recent polls.

The strategy in Georgia this weekend seemed to be to take his own political brand out of the equation, asking key voters to instead consider what could happen if Mr. Trump wins.

“He’s clearly unhinged,” Mr. Biden said while talking about a recent interview granted by the former president. “Buy Time magazine this week. Take a look at what he has said. He said, ‘A lot of people liked it when I said I would be a dictator on Day 1.’”

Earlier in the day, Mr. Biden also took a swipe at the recent polling. “You hear about how, you know, we’re behind in the polls,” he said. “So far the polls haven’t been right once. We’re either tied or slightly ahead or slightly behind, but what I look at is actual election results and election results are in the primaries.”

He added that Nikki Haley, who is no longer in the race, peeled away votes from Trump in several primary elections.

“It’s not about me,” Mr. Biden told a group of supporters, including several Morehouse graduates, gathered at a popular restaurant in Atlanta. “It’s about the alternative as well.”