What it says on the title. Apparently the SSRs were created as a measure to alleviate nationalistic pressures within the early USSR which somewhat gave way to the narrative that it was an "unbreakable union of freeborn republics" ( just with one considerably bigger and more poweful than all others put together, of course).
Of course that didn't work out, as the USSR broke apart on nationalistic lines. Your challenge would be to avoid that nationalistic structure altogether, bonus points if the capital is outside of Russia, in a separate DC equivalent.