Charlotte, North Carolina (CNN) President Barack Obama made his 2016 debut on the campaign trail Tuesday, deploying the same star power that he used eight years ago to defeat Hillary Clinton to now make a powerful case for his former rival -- all just hours after FBI Director James Comey brutally slammed Clinton's State Department email practices.

Obama appeared onstage with Clinton in Charlotte after lending her a ride from Washington on Air Force One and sought to confer his popularity on a candidate still struggling to gain voters' trust.

Several times, Obama rallied the crowd using his signature campaign chant from 2008: "Fired up, ready to go!"

In remarks that highlighted the unique political charisma that helped then-Sen. Obama win the White House, Obama touted the four years Clinton served as his top diplomat as evidence of her tough-nosed grit. He also acknowledged that he became a loyal Clinton convert years ago, and that his 2008 Democratic primary fight against his friend is all in the past.

"I came away from that primary admiring her even more because during that year-and-a-half, I had a chance to see up close, just how smart she was and just how prepared she was, especially since I debated her a couple dozen times," Obama said. "I saw how even when things didn't go her way, she'd just stand up straighter and come back stronger."

Obama never once mentioned Clinton's Republican rival, Donald Trump, by name. But the President appeared to thoroughly enjoy his return to the campaign trail as he repeatedly slammed the billionaire by going after his "tough talk" and "phony bluster."

"I know the other guy talks about making America great again. America's really great," Obama said. "This is a choice between whether we are going to cling to some imaginary past or whether we're going to reach for the future."

In a reference to Trump's past role as host of the realty TV show, "The Apprentice," Obama quipped: "This is not a reality show. This is not entertainment."

When the crowd here began to boo at Obama's mention of Trump's policies, the President urged: "Don't boo. Vote!"

Clinton, too, reflected on her first failed campaign against Obama.

"I feel very privileged because I've known the President in many roles. As a colleague in the Senate, as an opponent in a hard-fought primary, and the President I was so proud to serve as secretary of state," Clinton said, who spoke first next her one-time boss.

"I also know him as the friend that I was honored to stand with in the good times and the hard times. Someone who has never forgotten where he came from," she said.

Taking a jab at Trump and the conspiracy theories he has espoused about Obama's birthplace, Clinton said: "And Donald, if you are out there tweeting, it is Hawaii."

Clinton aides say the Charlotte Fire Marshall told them there were 7,000 people in the main hall and 2,500 people in the overflow room, making this Clinton's biggest event of the campaign.

But what should have otherwise been a celebratory moment for Clinton was partly overshadowed -- yet again -- by a scandal that continues to dog her campaign. Comey announced Tuesday morning just hours before the Clinton-Obama joint appearance that the former secretary of state's use of a private email server during her tenure at the State Department was "extremely careless."

Though Comey said that he would not recommend charges against Clinton, the long-awaited announcement -- coming after Clinton's lengthy interview at the FBI headquarters over the weekend -- was a striking rebuke of the Democratic Party's presumptive White House nominee. Even with the federal probe wrapping up, Republicans continue to seize on the email controversy as Clinton seeks to shed the perception that she is untrustworthy and dishonest.

"Although we did not find clear evidence that Secretary Clinton or her colleagues intended to violate laws governing the handling of classified information," Comey said, "there is evidence that they were extremely careless in their handling of very sensitive, highly classified information."

With polls suggesting that more Democrats trust Obama than Clinton, the President will no doubt be one of Clinton's most powerful surrogates in the general election.

Starting with his first in-person call for Clinton's election on Tuesday, Clinton aides hope the second-term President will serve as a validating voice that can vouch for Clinton's character, fitness and qualifications. The President, Democrats believe, can rouse voters not yet clamoring for Clinton.

In recent weeks, Obama has grown increasingly vocal in his denouncement of Trump.

Obama's push for Clinton comes after months of waiting.

Like most Democrats, Obama expected his party's primary to conclude far sooner than it did. The Orlando terrorist shooting last month further postponed Obama's first campaign event, as the President grew increasingly revved up to appear alongside Clinton, according to aides.

Originally, Obama and Clinton were set to appear in Wisconsin, a state whose Democratic primary Sen. Bernie Sanders won handily. But with Sanders hinting that he'll soon offer Clinton his full-throated backing, the campaign opted instead for a stop in North Carolina, which Obama lost narrowly in 2012. The pair campaigned only blocks from the site of that year's Democratic National Convention.

Clinton's top aides believe North Carolina is the biggest must-win for Trump -- and their best chance to flip a state that Mitt Romney won in 2012. Clinton first deployed general election staff to North Carolina in late April and their first ads in the state began running in June.

Underscoring the importance both sides are placing on North Carolina, Trump also campaigns in the state Tuesday, appearing after Clinton during an evening event in Raleigh.

The Clinton campaign and the White House also believe Obama can help galvanize the state's large African-American population to vote in November. Defeating Trump will require turning out the coalition of young people, suburban women, and minorities that helped Obama win two presidential elections.

"We are going to fight for every vote in this state, and with this state, we're going to win it," Clinton said Tuesday in Charlotte.

Soon joining Obama on the trail is Vice President Joe Biden, who plans to stump for Clinton in his hometown of Scranton, Pennsylvania, later this week.

For the President, a Trump presidency has become a nightmare scenario that has the potential to undo much of the work he's focused on over the past seven years. As recently as last week, Obama was issuing executive orders in the hopes of institutionalizing the priorities of his administration.

In what's becoming a regular occurrence, Obama used a foreign press conference (in this case, in Canada) to denounce Trump as the antithesis of populism. It wasn't the first time Obama ripped into Trump abroad; less than a month after the billionaire announced his candidacy, Obama shamed Trump for his criticism of Sen. John McCain during a news conference in Ethiopia.

On Tuesday, Obama told supporters that there was no one better prepared to fight for Americans than his former rival.

"I am ready to pass the baton," Obama said. "I know that Hillary Clinton is going to take it and I know she's going to run that race."