I accidentally posted an unfinished version of this, deleted the comment.

Iowa became really big in presidential elections after the 1976 election. The election was very similar to the current 2016 race because after Watergate and Ford's assumed vulnerability, there were 17 candidates for the Democratic nomination. Back then, it was believed that if you won the states with a lot of delegates (which usually had their primaries/caucuses in May or June), you would be more likely to win the nominate. Carter's campaign decided to go all in at Iowa (who had only moved their Caucus to January the previous election), going as far as to bus in their supporters to various events. This made the media catch on and start viewing Carter as having a lot more support than he really had, despite many more well established Democrats with more support running. Thanks to the momentum he got from Iowa and then New Hampshire, he was able to compete with the establishment and win the nomination as well as the presidency.

Here is a good article by Politico a few weeks ago that also shows each of the candidates Carter had to run against. It's a good read, especially for anyone wanting a historical precedent for what the Republicans are dealing with right now.