The two candidates clashed on lifting the cap on taxable income.
Sanders defended his view that Israel’s response in Gaza in 2014 was disproportionate.
The Democratic debate audience booed Sanders as he initially avoided the question.
Sylvan Lane 04/14/16 09:58 PM EDT
"Oh my gosh, they must have been really crushed by this," Sanders quipped.
"This is not just an attack on me, it’s an attack on President Obama."
"Dr. Song's comment was inappropriate and insensitive," Sanders wrote.
Transport Workers Union Local 100 says it represents 38,000 members and 26,000 retirees.
She will also be endorsed Wednesday by the New York State Immigrant Action Fund.
Support for the two Democratic candidates is even at 48 percent.
Clinton consistently polls double digits ahead of Sanders in New York.