Matt Yglesias, not known as a Republican booster, defends Mitt Romney's tax rate.
The main reason Romney's effective rate is so low is that the American tax code contains a lot of preferences for investment income over labor income. That's something that strikes many people as unfair on its face, and particularly unfair since it often means very low rates for extremely rich people like Romney. And Romney himself as a rich guy who's also a member of the political party seen as favoring the rich, and who's been recorded as whining that the working poor are undertaxed is perhaps not an ideal messenger for a defense of this policy.
But this is definitely an issue where the conservative position is in line with what most experts think is the right course, and Democrats are outside the mainstream.