They had gathered to oppose a plan by the federal government to bring its deportation muscle to Chester, a hamlet of 4,000 residents in the heart of a Republican stronghold 60 miles northwest of Manhattan. The idea was to retrofit a former Pep Boys distribution center into a detention facility that could confine as many as 1,500 migrants. But in a moment that foreshadowed the nation’s souring view of President Trump’s enforcement tactics, the proposal ignited instant bipartisan dissent.