US Democratic candidates attack Trump as Democratic presidential debate opens

Democratic presidential contenders attacked Republican President Donald Trump as a demagogue who is ripping apart the country and promised to restore unity during the opening minutes of a debate on Wednesday.

On the second of back-to-back nights of Democratic debates, front-runner Joe Biden was flanked at centre stage by rivals Kamala Harris and Cory Booker, US senators who are the most prominent black contenders in a Democratic nominating contest in which race has played a prominent role.

Biden cited that diversity on stage in his opening statement.

"Mr. President, this is America, and we are strong and great because of this diversity, Mr. President, not in spite of it," he said to Trump.

"Let`s get something straight, we love it, we’re not leaving it, we’re here to stay and we’re certainly not leaving it here to you," said Biden, who was vice president under America`s first black president, Barack Obama.

Seven other candidates, including former US Housing Secretary Julian Castro, Washington Governor Jay Inslee and US Senator Kirsten Gillibrand, joined them on stage in Detroit in search of a breakout moment that will generate new momentum for their campaigns.

The crowded Democratic field of about two dozen candidates has been vying for attention and financial support in the race to pick a challenger to Trump in the November 2020 election.

"Tonight, we have to get to the heart and soul of who we are as Democrats," said New York Mayor Bill de Blasio in his opening statement. "There are good people on this stage but there are real differences."

Biden did not figure prominently in the first night of the debate on Tuesday, which featured a clash between progressives Bernie Sanders and Elizabeth Warren and their more moderate rivals on issues such as universal healthcare and immigration.

But he was in the spotlight on Wednesday, after engaging for weeks in an escalating fight with Harris and Booker for the support of black voters, a vital constituency in the Democratic nominating battle.

Biden admitted he was taken by surprise last month when Harris criticized him for opposing federally mandated busing for school integration in the 1970s and for working with segregationists while serving in the US Senate decades ago.

The sharp attack gave Harris a bump in the polls, and Biden`s feeble response led to a dip in his standing. He has recovered in some recent polls and still maintains a firm grip on first place, helped by strong support from black voters. 

During the candidate introductions on Wednesday, Biden told Harris to "go easy on me, kid." His comment was picked up by a microphone.

The debates come about six months before Iowa holds the first nominating contest, and the two nights in Detroit could be the last chance for many of the lower-tier contenders to make an appearance on the national debate stage.

The Democratic National Committee will double its fundraising and polling requirements to qualify for the next debates in September and October, leaving out more candidates and raising questions about their ability to continue their campaigns.