WASHINGTON — Emboldened by President Donald Trump’s campaign platform of law and order, militia groups have bolstered their strength before Election Day by attracting military veterans who bring weapons and tactical skills viewed as important to the organizations. The role of veterans in the newly proliferating militia groups — which sometimes are steeped in racism and other times steeped simply in anti-government zealotry — has increased over the last decade, said a dozen experts on law enforcement, domestic terrorism and extremist groups.