They can't keep you or deny entry into the United States......they will just delay you and find out information about you if they wanted too.....but most agents won't hassle you......last year after the Baja 500 we drove across with my family and we all had out B/C and ID's and i told the agent we don't have passports, but we all had ID's and B/C and he said go ahead and go......never once did he ask to see them......