stage as the candidates face off tonight in the final gop debate of the year. nbc's peter alexander reports a highly anticipated showdown is expected on the vegas strip. >> reporter: the republican candidates are bracing for a freewheeling and fiery slugfest in tonight's final debate of the year, the first with ted cruz at number 2, taking shots from donald trump. the heated campaign season boiling over as the trump show rolled into vegas overnight. >> we should have been doing that for the last seven years. i give him credit. >> reporter: trump's contentious rally repeatedly interrupted by protestors. several forcibly removed and heckled by audience members, one even yelling a nazi salute. >> seil heil! >> reporter: in the five weeks since the candidates last shared the stage, the world has changed. paris and san bernardino fueling new concerns about terrorism and national security, igniting debates over syrian refugees and trump's proposed temporary ban on muslims entering the u.s. >> you have the radical islamic extremists, they're pouring in. >> reporter: all of it reshaping the

more from nbc news national correspondent peter alexander. >> reporter: sergeant bowe bergdahl breaking his silence. for years he was america's only prisoner of war after walking off his remote army post in afghanistan in the middle of the night. >> 20 minutes out, i'm going, good grief, i'm over my head. suddenly, it really starts to sink that i really did something bad. >> reporter: his first person account is in the new season of the podcast "serial." he wanted to warn commanders about leadership problems before concocting a new plan he might track taliban insurgents, casting himself as a super soldier. >> i was trying to prove to the world that i am, i don't know, jason bourne. >> reporter: critics argue bergdahl was a traitor, later swapped for five taliban detainees held at guantanamo bay. "serial" offering maximum exposure. >> the more information people have about sergeant bergdahl, his motivation, the better. >> reporter: 19 months after bergdahl's release in the white house ceremony with his parents, today house republicans argued the obama administration blind-sided lawmakers

more from nbc news national correspondent peter alexander. >> reporter: sergeant bowe bergdahl breaking his silence. for years he was america's only prisoner of war after walking out of his post in afghanistan in the middle of the night. >> suddenly, it really starts to sink that i bad. >> reporter: his first person account is in the new season of the podcast "serial." commanders about leadership problems before concocting a new plan he might track taliban insurgents, casting soldier. >> i was trying to prove to the world that i am, i don't >> reporter: critics argue bergdahl was a e guantanamo detainees. "serial" offering maximum exposure. information people have about sergeant motivation, the better. >> reporter: 19 months after bergdahl's release in the white house ceremony with his parents, today house republicans argued the obama administration blind-sided law makers with the prisoner swap. >> the administration intentionally violated the law not informing congress and misleading congress about these negotiations. >> reporter: to this day, bergdahl is haunted by his years