donald trump nominee head department homeland security tell u.s. senate committee wednesday need build wall mexican border sea shin sea remark contrast president campaign pledge kirstjen nielsen repeat confirmation hear predecessor john kelly say project kelly nielsen former boss step head department year trump chief staff build wall along u.s.-mexico border one trump central campaign promise far prototypes build fund remain question nielsen 45 serve kelly deputy white house chief staff secretary homeland security open months trump presidency response question senators homeland security committee nielsen say would work state local actors grind find best solutions border security include increase use technology unman surveillance aircraft rank democrat committee senator claire mccaskill raise concern cost-benefit analysis do need wall compare homeland security initiatives face cut president budget decisions base politics mccaskill say hear nielsen respond decision resources need risk base confirm senate nielsen would take rein department 240,000 employees responsible issue u.s. border airport security immigration policy disaster response refugee admissions cyber security expert highlight concern open remark cyber criminals nation state continually look ways exploit hyperconnectivity nielsen say day hear bipartisan group dozens former u.s. homeland security officials send letter support nomination nielsen serve administration republican president george w. bush special assistant president homeland security involve revise disaster relief policies hurricane katrina 2005. ask another democratic senator whether believe humans primary cause climate change scientists link worsen natural disasters nielsen say absolutely believe climate change many contributions homeland security nominee say need full u.s.-mexico border wall