Scenes of masked men toting guns and waving black Islamic State flags. Refugees scrambling across the border. Fires and explosions. It’s not just a Donald Trump ad. Most of the Republican presidential contenders and their allies are now waging campaigns focused on fear — bombarding voters with ominous television spots that warn of national security threats and amping up their alarming rhetoric on the stump. The commercials saturating the airwaves in Iowa and New Hampshire reflect how worries about terrorism are crowding out domestic issues such as tax policy and health care among GOP voters. The candidates are scrambling to outmuscle one another, offering dark assessments of the Obama administration’s fight against violent extremists and warning that their rivals are ill-equipped to take up the cause.