WASHINGTON — President Barack Obama authorized on Friday a broad expansion of the U.S. military mission in Iraq that could boost the total number of American troops there to 3,100 and spread advisory teams and trainers across the country, including into Anbar province where fighting with Islamic State militants has been fierce. The president's decision to escalate the U.S. effort in Iraq comes just three days after midterm elections that were bruising for Democrats and amid persistent arguments that more U.S. troops are needed to bolster the struggling Iraqi forces. In particular, the Iraqi government, members of Congress and others have called for troops in Anbar in western Iraq, where extremists have been slaughtering men, women and children.