How do you determine which candidate to support?

In writing this post I started discussing the process of deciding which candidate to support. I posed the question: "How do you decide which candidate merits your support?" I really don't mean it as a trite question, I've put a great deal of thought into it personally.

It used to be an easy decision for me. I used to be a religious Republican, which meant that I made by decision by listening in church for cues as to who everyone else was supporting. Occasionally, this would come from the pulpit, but more often casual discussions with other church members would lead to a kind of informal consensus as to who the 'best' candidate would be.

Now that I'm neither religious nor Republican, I'm not sure how people decide to support any given candidate, especially when parts of their ideology may conflict. For example: if you love candidate A's positions on domestic policy, but hate their foreign policy, do they still get your vote? Or substitute any set of issues you want, the question remains: in the absence of a perfect candidate for you, how do you go about prioritizing the issues so you come up with an acceptable candidate (or not) to vote for?

To me, I'm really struggling with this issue. I've spent the last 10 or so years throwing out or otherwise evaluating the old heuristics I'd use. Heuristics are especially useful or especially misleading when it comes to elections. For example- Mike Huckabee is touting his Christianity. For many people, this provides a useful way to shortcut all of his position issues and just give them a "yes" or "no" for supporting him. Heuristics are mental shortcuts based on past experience. So, if you are a Christian, this label attached to Huckabee is a shortcut way of expressing that he probably shares your values on issues such as abortion, evolution, science policy, stem cells, gay rights, etc.... You don't (mentally) need to take the time to actually evaluate his positions on each of these issues. It works the same way for me when I go to evaluate Mike Huckabee, except those shortcuts all lead to a "no" answer when I question whether our values are similar. For other Christians, Mitt Romney's Mormonism might be another heuristic for "no", even though they would probably agree on all of the issues. This is why I say they can be especially misleading.

These heuristics are great once the nominating stage of the race is over with and the parties have chosen their candidates. Most people just use their "party" heuristic at this point and vote for the person that the party as selected to represent them. However, at the nominating stage, one has to look a little bit deeper to find one's cues.

So here's where I stand: I'm 29 years old, white, middle-class, atheist, and a libertarian. I don't know much about how our foreign policy ought to look, I just know it's not working now. I'm not sure if non-intervention (a la Ron Paul) would be salvation or disaster. All of this means that there's no obvious candidate that I should support. Fiscally, I probably should side with a Republican candidate. Socially, probably a Democrat. Foreign policy gets a big "?". So how do I prioritize these? I think all of these issues are important-- if not for me personally, then for the country as a whole. Should I vote strictly my interests? Should I vote for the best interest of the country? How do I weigh these if they diverge? I was leaning towards Ron Paul lately, until I read this blog post at Daylight Atheism. So I think where I'm left is looking for Giuliani. We probably agree on economic policy, he's pretty liberal on social issues (gay rights, etc...) and I don't think he's looking to push his religion down anyone's throats. I don't plan on traveling out of the country any time soon, I don't know anyone that lives out of the country, and I don't know anyone in the military, so do I completely ignore foreign policy? That doesn't seem right, but what am I to do?

If you have any input, feel free to leave it in the comments. How are you deciding? What factors are the biggest for you?