DURHAM, NEW HAMPSHIRE (Reuters) - US Democratic presidential nominee Hillary Clinton shared a stage with former rival Senator Bernie Sanders on Wednesday (Sept 28) to appeal for youth votes in the Nov 8 election as opinion polls show a close race with Republican Donald Trump.

Clinton told an audience at the University of New Hampshire that she would make college affordable if she wins the White House, the kind of promise that won Sanders many young supporters during the Democratic nominating contest.

"We should and we will make public colleges tuition-free for families earning less than US$125,000 a year," Clinton said. She vowed to help those who already have student debt to refinance.

Clinton's campaign is worried that some polls show voters under the age of 30 might not turn out in great numbers at polling stations in November, potentially giving an advantage to Trump.

Members of the crowd on Wednesday waved signs that read: "I will vote."

Recent opinion polls have shown the race tightening between Clinton, a former secretary of state, US senator and first lady, and Trump, a New York real estate magnate.

A majority of Americans say Clinton won Monday night's presidential debate, but her performance does not appear to have boosted support among likely voters, according to a Reuters/Ipsos national tracking poll released on Wednesday.

The online poll found that 56 per cent of American adults felt Clinton did a better job, compared with 26 percent who believed the Republican did better.

Even so, Clinton's performance seemed to have little impact on her support. The poll showed 42 per cent supported her, while 38 per cent backed Trump.

Trump, often described as racist by Clinton, tried to turn the tables at a rally in Waukesha, Wisconsin.

He pointed to the Democrat's remark that "implicit bias is a problem for everyone, not just the police," when asked at the debate whether she believed police are implicitly biased against black people. "She accuses the entire country, including all of law enforcement, of 'implicit bias,' essentially suggesting that everyone, including our police, are basically racist and prejudiced," Trump said.

PRAISING SANDERS

Clinton's event with Sanders took place on a university campus, but it was not open to students without an invitation, according to attendees, many of whom were middle-aged and said they were members of local Democratic organizations or invited by the campaign.

Clinton praised Sanders, a US senator from Vermont who was her opponent in the hard-fought struggle for the Democratic nomination earlier this year.

"He is one of the most passionate champions for equality and justice that I have ever seen and someone that I am looking forward to working with," Clinton said of Sanders, who introduced her on Wednesday.

Although Sanders lost to Clinton, he consistently drew younger voters to his side with promises to take on Wall Street, make college less expensive and close the income gap.

He called on young people in New Hampshire, a swing state in the presidential election, to get behind Clinton. "Get your uncles, your aunts, get your friends to vote for Hillary Clinton," he said.

Clinton's campaign said it hoped to get Sanders to make more appearances on Clinton's behalf before the election.