Bush criticized Trump's claims about Sept. 11 and the treatment of a protester.
Sanders and O'Malley took part, taking questions from the progressive group.
Complaints were also filed to keep Marco Rubio and Ted Cruz off the ballot.
The Republican presidential candidate timed the surgery so he wouldn't miss any campaign events.
Trump is feuding with historians over Civil War history near one of his golf courses.
"Our lack of leadership is now creating chaos all over the world," Bush said.
Julian Hattem 11/24/15 01:17 PM EST
“This is eerily reminiscent of the president’s characterization of them as the ‘jayvee.’ "...
The move signals a heightened focus on other early-voting states.
"We've seen this before ... [and] it doesn't end well."
This is her 15th national union endorsement, compared to two for Bernie Sanders.