First, an Iowa win shows Trump can translate polling support into votes on the ground. The Iowa caucus is a notoriously difficult process to dominate with just charisma or ideological fervor. It usually takes organizational skills.
Second, an Iowa win will show Trump can come from behind, in this case, from behind Ted Cruz, with campaigning and advertising to add support in the face of strong competition.
Along with uncertainty of Republican caucus goers actually standing up for Trump in the February 1st meetings, there has been doubt that Trump can grow his support. If he does win the public religious boost will be enormous. Only Cruz can stop him as of today.
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