Labour leader says he wants to teach GOP presidential frontrunner about multiculturalism.
The third Democratic debate will air live on 19 December from New Hampshire.
Democrat front-runner challenges Republicans on gun control and terrorism.
Democratic presidential candidate comes ahead of US President Barack Obama and Pope Francis.
Debate on CBS News kicked off in Des Moines, Iowa after a moment of silence for victims of the Paris attack.
The CBS News/Des Moines Register sponsored debate is scheduled for 14 November at 9pm EST/2am GMT.
It is the second biggest labour union endorsement the Vermont senator has received for his campaign.
Turing boss Martin Shkreli bought the rights to the Daraprim drug for $55 million.
Sanders is drawing large crowds at campaign events nationwide.
Clinton and Sanders face off against lesser known candidates O'Malley, Chafee and Webb.
Rival Bernie Sanders closes the approval gap by 10% in the race to replace Barack Obama.
Congressman Raul Grijalva is the first member of Congress to endorse Sanders, who is running against Hillary Clinton.
Vermont Senator Bernie Sanders addressed a crowd of about 20,000 during a rally in Boston on 3 October.
Following a stop in Springfield, Massachusetts, the Vermont senator attracted 20,000 people in Boston.
Sanders has come to Boston as he continues to rise among the Democratic candidates for the 2016 presidential nomination. The university-filled city is the Vermont senator's second stop in Massachusetts on 3 October.
From Israel to Argentina, reaction ranged from concern to jubilation.
The Vermont senator leads Clinton by double digits in both key states.
Clinton was favourite to be Democrat candidate in 2016 but the Vermont Independent is taking over.
"Inequality is a drag on our entire economy," Clinton said.
New data released by Facebook show the two candidates are outpacing Republicans in Iowa and New Hampshire.
Young backs Vermont Senator Bernie Sanders
The Democratic candidate's team said the piece was 'a dumb attempt at dark satire'.