That is, in a state like California, whose Congressional delegation is overwhelmingly likely to be majority Democrat, this policy does indeed favor the Democratic Party. However, many states with high levels of illegal immigrants have GOP-dominated Congressional delegations. Increasing those delegations has the opposite benefit. Which party benefits the most overall? I'd say probably the Democrats, but it woudl be interesting to get real data.

In any case, the point should become moot: only citizens and legal foreign residents should county towards the population for the purposes of determining a state's number of Representatives.