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I totally agree. It sounds like he's starting to resent the fact that he has to support his parents in this way. But he can't just come out and say that because they, you know, raised him and all.
So he projects the resentment he feels onto an issue that feels more...worthy of it, I guess. By resenting them over the Republicanism issue (which goes to the core of who he is) rather than the money issue (which is just money), the resentment feels more justified and proper.
But there's no need for that. It's ok to resent being used in this way. It's ok to resent your parents, particularly if you feel like they (or your siblings) are taking advantage. And it's ok to deal with that directly, even if it hurts the relationship at first. I'd argue that it's better, even -- resentment is poison, and will just eat at your feelings over time. Better to have it out, get it over with, and heal.