Perhaps my students are not as cynical as I am but as soon as I know what a person's political beliefs are, it affects how I perceive what they say about policy issues, particularly if I do not agree with them. That is, I believe it's human nature for us to more readily accept information that is consistent with the world view we already hold, and to distrust information that is inconsistent with that world view. Dismissing information we don't like is easier when it comes from someone we can label as 'biased' (though it is also human nature to believe that people we disagree with are 'biased' while those we agree with are 'neutral').
In an economics class, even positive analysis can be perceived as normative if the conclusions are at odds with a student's worldview. A good example is tax incidence: every (neoclassical) economist in the world will tell you that it doesn't matter who the government collects the tax from (statutory incidence), both the seller and the consumer will pay the tax in the form of changed prices, relative to the price without the tax (economic incidence). But when I tell students that removing the gas tax does not mean the price of gas will fall by the full amount of the tax, they have a hard time believing it - even if they can follow the theory, they simply don't believe it. I worry that if they thought I was a "tax-loving liberal", they would probably be even less inclined to believe the theory itself. You can imagine how this problem would be compounded when we get into topics that are even less clear-cut.
So I bend over backwards to make sure that I am staying as objective as possible. When there are normative judgments to be made, I tend to talk in 'if-then' statements: "If you believe in the ability-to-pay principle, you would be more likely to support a progressive tax system" or "if you believe that the value of the benefit externality is large, then you might feel government intervention is appropriate", etc.
All of which is lead-in to the point of this post: I am considering using this McCain clip as part of a discussion of why we have a progressive income tax system. On the one hand, I think it sums up the basic issue pretty well (i.e., lots of people think it's unfair to tax rich people more, lots of other people think it's OK to ask people to pay more once they reach a 'certain level of comfort'), and I think that the fact that it's coming from John McCain could potentially give it more credibility with students who are Republicans. On the other hand, deep down, there's a part of me that gets some joy from showing those Republican students that the man is saying the exact opposite thing today from what he was saying 7 years ago. Because of that, I probably won't use it. But if it were anyone but McCain, I would, and that's bugging me too.