I would take a look at:
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their supposed positions on issues you care about
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how closely their actual voting and bill introduction record matches this
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their general demeanor and style of handling things / their reaction to local issues as legislators beyond voting on bills - being President is about taking action and responding to things, not just stances on certain issues
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their record of integrity - you want someone who is not going to waste a lot of our time with various scandals and sanctions
I can definitely see where you are coming from. I think a lot of the issues we care about aren’t particularly relevant to the presidency. Gun control and abortion, for example, are pretty much in a holding pattern and any major changes would be more a matter for congress or the courts than the president.
Even if McCain nominally holds the same positions, he seems to change them nilly willy now that the campaigning has started, and when interviewed doesn’t seem to be able to articulate why he is changing or even that he really understands the issue or his own point of view. This seems ripe for him to change policy once elected to suit whoever he thinks he is answering to.
Obama occasionally seems to change positions (like FISA) but is able to articulate well and explain the nuances of the situation and why he made the decision or why it’s a myth that he actually changed his position (like federal campaign funds).