Or just ask loud enough so that the potential-asshole and his two neighbors hear, giving you a 99.9% chance of getting a seat, and a 0% chance of getting chewed

Or just ask loud enough so that the potential-asshole and his two neighbors hear, giving you a 99.9% chance of getting a seat, and a 0% chance of getting chewed out because no-one acts like a dick to a pregnant woman on a crowded subway for obvious reasons.