Bernie Sanders has apologised to fellow 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 clashed over Syria, with Sanders accusing Ms 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.
Sanders 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.
These differences aside, the BBC's Laura Bicker at the debate says the candidates are not seeking a damaging race and used their TV appearances to appear as safe adults serious about policy in comparison to their Republican rivals.
Both 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 radicalised.
All three candidates said it was important to work more closely with Muslim-American communities to tackle radicalism at home.
Bernie Sanders admitted that on two occasions his campaign could see proprietary data from Hillary Clinton's campaign following computer breaches - which he said were the fault of the software vendor.
He said that the most recent breech involved inappropriate behaviour by one of his staff members, adding that person had now been dismissed.
He said that the Democratic Party's decision to temporarily suspend his campaign's access to the strategically crucial database was "an egregious act".
The Sanders campaign on Friday filed a lawsuit against the Democratic National Committee in a federal court to restore its access to the voter data.
Strategically important information on voters is contained in the database, which campaigns use to decide strategy.
That data takes on a crucial role as campaigns prepare for early primary voting in just over a month's time.