The case for adjustment rests on a curious notion of political power, one that reduces political muscle to sheer numbers. Obviously, numbers are important. But even more important is what people do with those numbers. In essence, adjustment advocates argue that population totals ought to translate into power independently of political effort. In an era of concern about excessively thin notions of civic obligation, this is a strikingly anemic notion of citizenship.
Republicans are loath to seem opposed to minority empowerment and rights, especially voting rights.
As for Democrats, they've focused on the undercount because it's one issue for which they believe they have a simple remedy. Like redistricting, adjustment is a highly technical issue not likely to arouse much opposition among the general public. And whatever costs it might generate can be postponed for a few years, perhaps indefinitely. Unfortunately, the particular palliative of census adjustment is a cure worse than the disease. The only group that adjustment would truly empower would be the demographers and statisticians directly involved with it.