the talk is nonsense. >> the son of a u.s. citizen born abroad is a natural-born citizen. >> reporter: he told cnn's dana bash he's done addressing it. >> i'm not going to engage in this and the reason is simple. there are far too many serious issues facing this country. >> reporter: one more sign the fault line in the republican primary are becoming clear. it's not only trump versus cruz but chris christie versus marco rubio. christie said rubio is too weak to be the nominee. >> is that the person that we want to put on the stage? >> reporter: rubio fired back comparing his record to barack obama. >> all of the damage barack obama has done to america. >> reporter: and donald trump is moments away from taking the stage here in burlington, vermont. you can see the crowd gathered behind me. you may wonder why here.