Trump: "Nothing wrong with listening" if Russia or China offers dirt; Trump says he might not tell FBI if foreign govt. offered info; Sen. Warner introduced bill requiring candidates to notify FBI if contacted by a foreign agent; Sources: In private conversations, Sanders has discussed running for Governor of Arkansas; From defending to denouncing: GOP splits on Trump comments; Trump calls out FBI Dir.: He is "wrong" to say candidates should report report foreign govt offers dirt on opponent; Trump: All congressmen take "oppo" research from Govts.; Dems slam Trump as "UN-American," "Disgraceful," "Lawless"; Dem candidates outraged over Trump saying he'd take dirt from foreign govt via Knit