Dave, for some, any consideration of a GOP candidate could easily be precluded early in the process. For example, the fealty of the GOP to the hard right, particularly social conservatives, would have a considerable impact on the court appointments that a president will make. The Supreme Court is already unbalanced toward the ultra conservative justices. Because of their relative youth (for Supreme Court Justices), the potential is there for a GOP president to put the SCOTUS out on the right end of the spectrum for years to come. That is an extremely important consideration in my decision about a presidential candidate.
Also, although from a world view, there might not be that much difference between the parties in terms of economic theory, there is enough for me to make a choice between candidates based on party affiliation. The chamber of commerce crowd holds considerably more sway with the GOP than with the Dems when it comes to labor-protective legislation and regulation. None of the restrictions on corporate power that are needed would have a prayer in a GOP administration. It would be a tough enough hill to climb with the Democrats in power.
In short, people can have sound reasons to choose party over candidate in a presidential election. It is not because of a lack of thought that one would do so.
Then I'll get on my knees and pray,
We won't get fooled again; Don't get fooled again