What each candidate needs to do in the Iowa caucuses

In January 1980, George H.W. Bush narrowly won the Iowa caucuses over front-runner Ronald Reagan. Bush had trailed Reagan by 9 percentage points in the final poll of the Republican contest, but he triumphantly beat expectations. Having won, Bush claimed he had the "Big Mo" — momentum — moving forward. And while Bush didn't win the GOP's presidential nod, his Iowa performance put him on course to become Reagan's main rival, and later, the vice presidential nominee.

As the 1980 case demonstrates, Iowa has helped shape the results of presidential...