Political parties have one primary goal and one primary goal only - to win elections. Republicans used to understand that. Democrats used to be the ones who wanted to "fall in love" and "stand for principle." What do you call a candidate who loses on principle? You call them a loser.
Sure the parties themselves have that goal, but the people in them, the voters, need a reason to actually go out and vote for the party, campaign for the party, and donate money to the party. If it's just so team Atlas blue can win instead of team Atlas red and you don't see any real difference between the two, why bother? Sure the political junkies of the world who love politics' for politcs' sake and want nothing more than to be in power will always stand by the party no matter what, but a lot of the people out there, in both parties' political base especially, are dedicated to their party because they really believe in the causes the party is advocating and think they have a real contrast in the two options before them. People need to see a reason to care who wins. For most people the party is a means to an ends and not an ends unto itself.
Now in the case of McDaniel or Murdoch, I don't agree with the means the base chose to try and reach its myriad of ends but in order to understand why they voted how they did you have to realize that they saw the establishment Republicans as functionally no different than Democrats so in their mind, however mistaken they might be, they saw Lugar as a 100% chance of losing and Murdoch as only a 50% chance of losing, weighing a Donnelly win or a Lugar win both as losses.
