Question

How has the mass media changed the presidential nominating process with horse-race coverage?

Answer 1

Because of the media, the people now had a complete profile from each nominees for the people to see  (like what displayed in a horse-race).
Since the media could choose which information to give to the audience, the media now held a very powerful influences that could determine the outcome of the elections.

Answer 2

The media has made it more important for candidates to be ahead early in the process, which allows for them to secure the funding and the volunteers necessary to continue the campaigns over the long time-frame the election season entails. Those who are unable to garner the attention of the media are typically shut out of the nominating process and have little-to-no chance of gaining the party's nomination at the convention.