By Rami Ayyub WASHINGTON (Reuters) – A Texas school shooting, a looming Supreme Court decision on abortion and grievances about security at the U.S.-Mexico border could mobilize American extremists to violence, the Department of Homeland Security (DHS) said on Tuesday. Violent threats directed at democratic institutions, politicians and political party offices will likely increase ahead of November’s congressional election, DHS said in its latest National Terrorism Advisory System Bulletin. It was the sixth such advisory issued by DHS since January 2021, when officials turned fresh attention t…