Airport security screeners could quit en masse, grounding flights. The federal courts could stop hearing civil cases. City buses could stop running. And 38 million Americans could stop getting food stamps. Officials from Washington to Wall Street are pondering nightmare scenarios if the partial U.S. government shutdown that is already the longest on record extends into spring — or beyond. “Shutdowns don’t get bad linearly; they get bad exponentially,” said Sam Berger, a senior adviser at the Center for American Progress, who worked at the Office of Management and Budget under President Barack Obama. President Donald Trump’s administration has found creative means to blunt some of the shutdown’s effects — figuring out ways to process tax refunds, for example.