They are able to get close because the Media allows them to set up in front with them, just so they can film the confrontation and have a story for the day. They are like-minded props for the Media...
Yell back louder.
I saw that guy on the FOX coverage while I was having lunch.
All the other attendees were holding Romney posters in front of him so he could not be seen from the stage. It look like he was trying to climb or stand on something to get a little above the crowd. You could not hear what he was yelling but his actions were clearly violent.
He will be back.