The problem with this is that we have no real record of their political or social structure. Everything we know about them currently is based on archeological analysis. While it seems increasingly likely that the symbols we see on their seals were a form of script, we have no idea how to read it or even what language family it pwould represent nor have we found any text longer that 36 symbols. So this means we don't have much to work with here.
So what can we glean from the archeology? The city layout was typically fairly even across the city with all residents benefitting from high levels of sanitation afforded by the sewage system. There seems to have been some social order as evidenced by a large amount of communal buildings, but the exact nature of this remains unclear. The various sites indicate a continuity of culture with regards to layout, production, technology, and script, but it is unclear if this meant that there was a central government or even a shared language.
If indeed there was a central body, Mohenjo-Daro and Harappa seem to be prime candidates due to their size and layout. Both sites lacked the city walls, had more spread out defenses and had much larger populations, but his is hardly conclusive evidence either way.
In any case, we know there was a great deal of trade between the IVC and other civilizations with seals being found in Ur and Elam. Gurthermore evidence such as large amounts of portable goods indicates a thriving intercity trade network.
So honestly, we know very little about the political and social structure. Everything from de-centralized republic, to oligarchic city states, to centralized kingship is on the table. Recent finds have indicated some degree of social stratification and we cannot rule out the possibility of classes of people now lost to time. It is likely we won't ever get as clear a picture as we have of Egypt or Sumer simply due to the relative parcity of material finds. Even if we decipher the Indus Script, we have no real texts and the seals are likely for trade.