A few days before the start of early voting, Michelle Vallejo, a 33-year-old Democrat, was greeting a dozen or so supporters in a shady corner of a municipal park in Edinburg, Texas, when she received a visit from an old friend. The volunteers, who had shown up to grab walk lists and water bottles for a pre-election canvass, were filling her in on the latest developments in their ongoing efforts to protect yard signs from being defaced and removed.