CHARLOTTE, N.C. — In their first joint campaign appearance, President Obama said Tuesday that he is “ready to pass the baton” — and the White House — to his former rival, Hillary Clinton.

Standing at a lectern bearing the presidential seal, Obama made the case for Clinton as a brilliant, “level-headed” stateswoman who could lead the country in tumultuous times.

As Clinton watched, the president explained how he came to admire Clinton as the two competed in a bitter Democratic primary in 2008, and why he ultimately decided to make her his secretary of state.

“I saw again and again how, even when things didn’t go her way, she just stands up straighter and comes back stronger,” Obama recalled. “She just kept on going. She was the Energizer bunny. She just kept on.”

Obama vouched for Clinton as a loyal hard worker who put her personal ambition aside to work for him. In a video released shortly before the appearance, Obama said Clinton acted like a “trooper” when she lost the primary to him eight years ago. In his speech, the president further praised her as “brilliant” and steady, telling the crowd in the Charlotte Convention Center that he’s “fired up” and ready to work to get her elected.

The Clinton campaign hopes Obama — whose approval rating is higher than at any time since he announced the successful conclusion of the operation to kill Osama bin Laden — can counter Clinton’s high unfavorable ratings and help unify the Democratic Party, which is still fractured after this year’s long primary campaign. Clinton’s rival, Sen. Bernie Sanders, has not conceded or endorsed her, even though she locked up the nomination weeks ago. Obama’s speech came just hours after FBI Director James Comey announced that his department would not be recommending charges over Clinton’s use of a private email server for official business while she was secretary of state. Neither Obama nor Clinton, who spoke before the president, addressed the news in their remarks.

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Obama did, however, tackle some of the objections that have been made against Clinton, including the argument that she has been in politics for too long. He said he benefited from being a fresh face in 2008, and that the country likes novelty. “We’re a young country, so we like new things,” Obama said. That means, he said, that voters may take for granted those who have “been in the trenches,” like Clinton. “The fact is, Hillary is steady, and Hillary is true,” Obama said.

“I’m ready to pass the baton,” he said at the end of his remarks.

Obama stayed out of the long Democratic primary until last month, after Clinton won California and clinched the nomination. He has been criticizing Trump for months, however — repeatedly slamming Trump’s proposed ban on Muslim immigration. His relaxed, humorous presence on the stump will complement the intensity of Clinton, who said earlier this year that she is not a “natural politician” like Obama.

Clinton addressed the crowd first, Obama sitting on a chair listening behind her, and described the pair’s transition from “political rivals to partners to friends.”

She said that no one who looked either like Obama or herself would have been included in the Founding Fathers’ vision of what a president should be, but that “we’re here today because the story of America is the story of hard-fought, hard-won progress.”

Clinton said she would focus on “building” on the progress Obama has made as president.

Both candidates also took several swipes at Donald Trump. Obama said that knowing how to use Twitter does not prepare a person to become president. “A bunch of phony bluster doesn’t keep us safe,” he said. Obama also remarked that the presidency is not a “reality show,” a reference to Trump’s show “The Apprentice.” “When a crisis hits, you can’t just walk off the set,” Obama said.

Clinton joked in her introduction that Obama is “someone who has never forgotten where he came from. And Donald, if you’re out there tweeting — it’s Hawaii.”

Trump, who was also campaigning in North Carolina on Tuesday, charged that Obama’s flight to Charlotte aboard Air Force One would cost taxpayers “a fortune.” The White House said earlier in the day that the Clinton campaign would offer a partial reimbursement for the political travel.

The real estate developer also criticized Comey’s decision not to bring charges against Clinton in the email case.

Taxpayers are paying a fortune for the use of Air Force One on the campaign trail by President Obama and Crooked Hillary. A total disgrace! — Donald J. Trump (@realDonaldTrump) July 5, 2016

One supporter, Mary McCray, 63, a retired teacher who had waited with her two grandchildren in 90-degree heat to see Obama and Clinton, said she understood the evolution in the relationship between the two. The transformation Obama described in how he viewed Clinton from 2008 to now was familiar to several supporters who were waiting to see the pair.

“I felt at that time [in 2008], she wasn’t ready,” said McCray, who backed Obama that year. “I felt like it wasn’t her turn yet.”

McCray says she is now enthusiastically supporting Clinton, in part because of her service as secretary of state in Obama’s administration.