WASHINGTON — A looming Supreme Court decision on abortion, an increase of migrants at the U.S.-Mexico border and the midterm elections are potential triggers for extremist violence over the next six months, the Department of Homeland Security said Tuesday. The U.S. was in a “heightened threat environment” already, and these factors may worsen the situation, DHS said in the latest National Terrorism Advisory System bulletin. “In the coming months, we expect the threat environment to become more dynamic as several high-profile events could be exploited to justify acts of violence against a range of possible targets,” DHS said. It’s the latest attempt by Homeland Security to draw attention to the threat posed by domestic violent extremism, a shift from alerts about international terrorism that were a hallmark of the agency following its creation after the wake of the Sept.