PHILADELPHIA — Bernie Sanders loyalists warned that the Democratic Party could rupture over the nomination of Hillary Clinton after a volatile night that saw a large group of Sanders delegates and supporters exit the party’s national convention to stage a sit-in at a nearby media tent. They rejected Sanders’ call for unity even after the Vermont senator took the symbolic step of declaring Clinton the winner of the state-by-state delegate count inside the convention in Philadelphia. “I suspect we are witnessing an event that will fundamentally change American politics,” said Cory James, 22, a college student from Flint, Michigan, who expects the Democratic Party to break apart over Clinton’s victory. Thousands of activists have taken to the streets during the convention this week to voice support for Sanders, a liberal U.S.