It's a little of both. Nowadays, opposition to DC statehood tends to come from Republicans, who are motivated purely by electoral considerations, and who know

It's a little of both. Nowadays, opposition to DC statehood tends to come from Republicans, who are motivated purely by electoral considerations, and who know that DC, now being a largely Black city, will vote Democrat. (Not considered: adjusting the Republican platform to be more appealing to Black voters. I digress.) But when it was originally decided that DC would not be a state, this had nothing to do with race. First of all, DC's demographics were different at the time; second, enslaved people couldn't vote anyway; and third, the founders were concerned that a national elite would form in DC that would tyrannize the rest of the country, so the point was to weaken that (hypothetical) faction. So it's ahistorical to blame DC's original non-statehood on racism -- it was a reasonably principled choice -- but the current reality (going back at least to, say, the '20s -- sometime post-Civil-War) does contain a lot of racial motivation. Ironically, the current situation is the inverse of what the Founders worried about: They thought a powerful elite would settle in DC and rule the rest of the country in an unrepresentative way. Instead, the people who live in DC are largely from an underclass in need of representation. So DC statehood now makes sense. But they couldn't have known that when they were setting things up.