LAS VEGAS – From the opening bell, Democrats unleashed an aggressive verbal assault on New York billionaire Mike Bloomberg and raised new questions about Bernie Sanders’ take-no-prisoners politics in a contentious debate Wednesday night that threatened to scramble even further the party’s urgent quest to defeat President Donald Trump. The former New York City mayor was forced to defend his divisive record on race, gender and Wall Street in his debate-stage debut, while Sanders, appearing in his ninth of the 2020 primary season, tried to beat back pointed questions about his health and his embrace of democratic socialism. Fierce exchanges throughout the 2-hour affair marked the most aggressive sustained period of infighting in the Democrats’ yearlong search for a presidential nominee, reflecting rising urgency in a 2020 primary season that is already deep into its voting phase.