Two days after Hillary Clinton held court with Boston Mayor Walsh and State Attorney General Maura Healey in Dorchester, U.S. Senator Bernie Sanders drew a estimated 20,000 people to his latest campaign stop in Massachusetts, according to staff.Hundreds of supporters filed down Summer Street to the Boston Convention and Exhibition Center to hear the white-haired Vermonter speak Saturday evening. A handful of button salesmen dotted the route, shaking off the October chill, while “Feel the Bern” could be seen scrawled in chalk on plywood all across Fort Point.A trio of speakers representing the key tenets of Sanders’ platform warmed the crowd—a reported 4,000 of which overflowed into a spillover room, according to Sanders’ campaign staff. Karen Higgins, co-president of National Nurses United, talked affordable healthcare. Jimmy O’Brien, president of the Boston Carmen’s Union ATU, took a shot at Governor Charlie Baker’s efforts to privatize some MBTA services.