It is the scenario that President Trump’s most conservative followers considered their worst nightmare, and on Wednesday it seemed to come true: The dealmaking political novice, whose ideology and loyalty were always fungible, cut a deal with Democrats. If Mr. Trump’s agreement with the two Democratic leaders, Senator Chuck Schumer and Representative Nancy Pelosi , to increase the debt limit and finance the government for three months did not yet represent the breaking point between the president and his core, hard-right base of support, it certainly put him closer than he has ever been to tipping his fragile political coalition into open revolt.