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 criticised Republican frontrunner Donald Trump, for his call to ban Muslims from entering the US, but they disagreed over Syria.
Mrs Clinton 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.
Mr Saunders, however, argued that the US should first concentrate on defeating the Islamic State (ISIS).
“Getting rid of dictators is easy, but you have to think about what happens the day after,” he said.
Former Maryland Governor, Martin O’malley, also took part in the debate in Manchester, new Hampshire.
The debate was the first for democrats since 14 people were killed by a married couple that the authorities said had been radicalised.
They clashed over Syria, with Mr Sanders accusing Mrs Clinton of being set on regime change while she said US leadership was needed.
Mrs Clinton, the former Secretary of State, remains the frontrunner.