President Obama told fans of Hillary Clinton in North Carolina he's ready to "pass the baton." He's hoping his political push will help Clinton across the finish line in the presidential relay race.

"I'm here today because I believe in Hillary Clinton," Obama told several thousand supporters at the convention center in Charlotte, N.C. "I want you to help elect her the next president of the United States of America."

Clinton hitched a ride with Obama to North Carolina aboard Air Force One. The joint appearance in the state could give Clinton's campaign a lift in what's likely to be one of the most hotly-contest battlegrounds of the fall campaign. It's also the first time in at least 100 years that a president has campaigned for his chosen successor.

Even before the blue and white 747 took off from Joint Base Andrews outside Washington, FBI Director James Comey had removed a potential storm cloud from Clinton's path. Comey announced the bureau will recommend that no charges be filed over Clinton's use of a personal email server for all electronic correspondence while she was secretary of state.

While Comey said Clinton and others were "extremely careless" in their handling of classified information — sending at least 110 classified emails through unsecured channels — "no reasonable prosecutor would bring such a case" in the absence of clear evidence that Clinton deliberately violated classification rules. Though details Comey gave about the investigation countered several of Clinton's original claims, including whether she sent or received classified messages.

The White House said it had no prior knowledge of Comey's announcement or even that he would be speaking about the investigation on the day of the long-planned campaign trip.

Clinton and Obama stepped off the presidential aircraft together—a potent political symbol that drew swift condemnation from Clinton's Republican rival, Donald Trump.

"Taxpayers are paying a fortune for the use of Air Force One on the campaign trail by President Obama and Crooked Hillary," Trump complained via Twitter. "A total disgrace!"

The Clinton campaign is expected to defray some of the cost of the flight, consistent with past practice.

At the convention center rally, both Clinton and Obama recalled the bitter primary battle they waged eight years ago, and the mutual respect that grew out of it.

"We may have gone toe-to-toe from coast-to-coast," Obama said. "But we stood shoulder-to-shoulder for the ideals that we share."

The Clinton campaign believes the evolution of that relationship makes Obama a powerful spokesman, especially with voters who still harbor personal reservations about Clinton.

"Hillary's got her share of critics," Obama acknowledged. "That's what happens when you've fought for what you believe in. That's what happens when you dedicate yourselves to public service over the course of a lifetime. And what sets Hillary apart from so many others is she never stopped caring. She never stopped trying."

The president highlighted Clinton's experience and preparation as a former diplomat, senator and first lady — and he tried to draw a sharp distinction with Trump, the presumptive Republican nominee.

"Everybody can tweet," Obama said. "But nobody actually knows what it takes to do the job until you've sat behind the desk."

Clinton also went after Trump, calling the businessman, "simply unqualified and temperamentally unfit to be president."

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