Bernie Sanders has apologised to fellow US Democrat presidential candidate Hillary Clinton after his staff stole valuable voting data from her campaign.‘This is not the type of campaign that we run,’ he said during a TV debate.The candidates criticized Republican frontrunner Donald Trump for his call to ban Muslims from entering the US.But they clashed over Syria, with Sanders accusing Clinton of being set on regime change while she said US leadership was needed.Former Maryland Governor Martin O`Malley also took part in the debate in Manchester, New Hampshire. But the former secretary of state remains the frontrunner.On Syria, she insisted that the US should seek to remove President Bashar al-Assad from power. ‘If the US does not lead, there is not another leader - there is a vacuum,’ she said.Saunders however argued that the US should first concentrate on defeating so-called Islamic State (IS). ‘Getting rid of dictators is easy, but you have to think about what happens the day after,’ he said.Both the main speakers had strong words for Trump, with Ms Clinton calling him ‘the biggest recruiter for IS’ and saying he used ‘bigotry and bluster to inflame people’.The debate was the first for Democrats since 14 people were killed by a married couple that the authorities say had been radicalized.All three candidates said it was important to work more closely with Muslim-American communities to tackle radicalism at home.