They also clashed on Wall Street banks, gun controls and the minimum wage.
Mr Sanders has won seven of the past eight contests, but Mrs Clinton holds a clear lead in race for the nomination.
The Democrats have largely avoided the personal attacks that have dominated the Republicans` debates.
But with so much at stake that changed in Thursday`s debate.
"Does Secretary Clinton have the experience and intelligence to be president? Of course she does," Mr Sanders said at the debate. "But I do question her judgement."
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