WATCH: Obama Jokes About Trump's Birther Reversal

President Obama is laughing off Donald Trump's sudden turn-around on his birth.

"There's an extra spring in my step tonight," Obama said to a dinner hosted by the Congressional Black Caucus Foundation in Washington, D.C., on Saturday night. "I don't know about you guys, but I am so relieved that the whole birther thing is over. I mean, ISIL, North Korea, poverty, climate change, none of those things weighed on my mind like the validity of my birth certificate."

After years of fanning false rumors that Obama wasn't actually born in the United States, Trump declined to answer whether he now believes Obama is a natural-born U.S. citizen in an interview with the Washington Post this week, as he has done throughout the campaign. That's despite claims in recent weeks from Trump's top campaign surrogates that he does believe the fact that the president was born in Hawaii.

Finally, on Friday, the Republican nominee held an event at the new Trump hotel in Washington, D.C., where he spent time talking up the new facility, introducing a number of veterans who've endorsed him, and then said, "President Barack Obama was born in the United States, period."

Obama addressed the controversy at the start of his speech to the CBC last night. "In other breaking news, the world is round, not flat," Obama added, echoing remarks he made during the 2011 White House Correspondents Dinner, where the president mocked Trump at length as he sat in the crowd with cameras trained on him.

Members of the Congressional Black Caucus responded swiftly and vigorously to Trump's comments on Friday. At a news conference in Washington, members focused on the apparent goal of the claims about Obama's birth to delegitimize his presidency, which is perceived by many as racially-motivated. The so-called "birther" movement largely launched Trump into the political arena, and CBC members made clear they didn't take his reversal seriously.

"This is not just about degrading the reputation of Barack Obama. It's about degrading the American dream for all African-Americans," said Rep. Gwen Moore, D-Wisc.

Democratic nominee Hillary Clinton also spoke at the CBC dinner on Saturday. "Mr. President, not only do we know you are an American, you're a great American. And you make us all proud to be Americans, too," Clinton said.

Clinton, who has seen her advantage in the polls slip away recently, is in need of enthusiastic support from African-American voters, as well as other demographic groups that helped to twice elect Barack Obama, including Hispanics and young voters.

"I need your help over the next 52 days," Clinton told the crowd on Saturday evening, as she encouraged them to register people to vote.

Clinton framed the stakes of the election in terms of protecting the Obama legacy.

The president, who noted that it was his last opportunity to address the annual CBC gathering while in office, spoke of the historical significance of his election as the first black president.

"We understood the power of the symbol. We know what it means for a generation of children, of all races, to see folks like us in the White House," Obama said. "And as Michelle says, we've tried to be role models, not just for our own girls, but for all children, because we know they watch everything we do as adults."

Obama also implored African-Americans to back Clinton with the same enthusiasm with which they voted for him: "You want to give me a good sendoff? Go vote."