I'm not sure it's true that
> states that normally vote Democrat are more likely to lose votes than states that normally vote Republican
because it could be true (1) that individuals who vote Democrat are more likely to go uncounted but (2) that states that vote Democrat are not more likely to be undercounted.
A few (totally imaginary) mechanisms that would do this: 1. Suppose Democrats who are surrounded by Republicans tend not to respond to the census, but everyone else does. Then you'll undercount Democrats, but mostly in places full of Republicans. 2. Suppose rural Democrats tend not to respond to the census, but everyone else does. Then again you'll undercount Democrats, but mostly in more-rural states, which tend to lean Republican. 3. Suppose poorer people (however they vote) tend not to respond to the census, but everyone else does. All else equal, richer people are more likely to vote Republican, but richer states are more Democratic. (I think Andrew Gelman's "Red State, Blue State, Rich State, Poor State" talks about this.) Then Democrats will be undercounted more, but Republican states may be undercounted more.
I don't know whether mechanisms with this sort of counterintuitive property are actually responsible for enough of the census-counting difficulties to produce that paradoxical outcome, but it doesn't seem obviously impossible.
(Of course, if what we're considering is the nefariousness of possible motivations, then what matters isn't whether such mechanisms actually do do that, but whether the potentially-nefarious people in question expect them to. That's a different question and my guess is that they probably don't even if they should.)