I spent far too long writing an answer, so I'm going to preserve it as an entry of my own.
People in general like to feel that they are making their political choices based on logic and sound rational reasoning, but in reality almost all of us make our choices based on emotional or intuitive factors and then look for logical arguments to back up the answer we wanted. This is more or less equally true of both left and right leaning voters. This means that a successful candidate is always actually appealing to emotions, values, and prejudices, but since we feel more comfortable about ourselves when we have a nice veneer of rationalization to justify our choice, it's also very useful to present a superficially logical case.
A truly logical case for a candidate is a very complex argument. It requires considering many issues, pulling out a lot of evidence, justifying each piece of evidence not merely in its superficial veracity but in its relevance to the whole argument and its importance relative to other evidence. Important pieces of contrary evidence cannot be ignored or lightly dismissed; they must be rebutted in detail. All of the pieces must be brought into a logical whole, and a case must be made that the goals the candidate is campaigning on are all achievable together even though they compete for finite resources.
In today's world of 30 second TV spots and two minute answers to questions even in debates, nobody has time for real, fully developed logic. If the candidate does make a real, logical case, at book length, almost no one will read it (though many will talk as though they did). Nobody genuinely wants logic. They want a simple, easy to follow case that feels logical.
Only now will I mention my own partisan opinion. The success of the Republican party in the modern political era (starting with the 1980 campaign) can be traced straight to their superior understanding and more effective embrace of the above ideas. Conservatives put a lot of effort into framing the terms of the debate and choosing the labels that will be attached to people and issues while tricking liberals into arguing about substance. (Why do we call the people who want to outlaw abortion "pro life"? Because the right knows how important labels are and they fought hard for it, while the left worried about the issue.) Then, once they've written the dictionary, they can keep reinforcing their emotional message with the semantic content of words. Because the emotional message is in the terms themselves, they can devote a lot of effort to giving their arguments the appearance of logic. They choose their evidence carefully, not for being the best evidence but for being the most convincing in a sound bite. Because they can state their evidence quickly and they don't need to explain it, they have more time to build the argument on top of it with superficial logic. It's actually more logical to present more evidence and explain it better, rather than focusing on the conclusion.
Republicans are very good at appearing more logical than they are.