For an alternate opinion on which way Chuckie will jump: Why Schumer will side with Obama on Iran deal.
But party loyalty is a euphemism for personal ambition. Schumer is now a macher, soon to become the most important and powerful Jewish pol in America. Yet that power ultimately requires conformity. He lost his right to be a rebel the minute he decided he wanted to be a party leader. Not that he was ever much of a rebel in any event. His reputation for being a motormouth camera hog notwithstanding, only very rarely does he go against Obama.
See also this UN idiocy: U.N. condemns Israel, U.S. for not sharing Iron Dome with Hamas