After months on the sidelines, President Barack Obama joined Democratic presidential candidate Hillary Clinton on the campaign trail at a rally in North Carolina on Tuesday, telling voters he was ready to "pass the baton" to his former secretary of state.

Obama took the microphone in Charlotte, chanting "Hillary!" and told the crowd there had never been a candidate as prepared to be president as Clinton, his rival in 2008 for the Democratic nomination.

"I've run my last campaign, and I couldn't be prouder of the things we've done together, but I'm ready to pass the baton," Obama said, in what was likely to be the first of many trips this year on Clinton's behalf.