SEATTLE — A leader of the Proud Boys, the far-right organization emboldened by President Donald Trump to “stand back and stand by,” said the group’s mission is to combat left-wing insurrection in the increasingly volatile culture wars leading to the November election. Joe Biggs, a Florida-based organizer for the male-only group known for street violence, said in a phone interview Wednesday that he was encouraged by Trump’s comments during Tuesday’s presidential debate when the president avoided condemning far-right and white supremacist groups. Biggs, who helped organize a rally Saturday in Portland, Oregon, said that condemning the social justice protests erupting nightly in that city is key to his group’s national strategy of thwarting Black Lives Matter and the leftist, anti-fascist movement known as antifa.