A brawl erupted between opponents and supporters of President Bush [related, bio] last night behind police barricades a block from his fund-raiser at the Park Plaza Hotel. Two Bush supporters, twin brothers Matt and Aaron Margolis, traded blows at the corner of Arlington and Boylston streets with several men who they said were union members. The fight started after a man wearing an Ironworkers Local 7 sweatshirt, perched atop a subway entrance, repeatedly taunted the brothers. ``They told us to get out of here or we'd get beaten up,'' said Matt Margolis, 24, of Beverly. ``I said, `I support your...