Because the Dravidian (or Harappan) language has not yet been deciphered; there is no way to determine their political structure, although the presence of two large cities with roads built on a grid, flush toilets and other amenities plus large city walls for protection indicate that some sort of political structure was present. Substantial public amenities and protective features could only be built by means of an effective system of taxation; therefore the Dravidians apparently had a system of taxation which one would naturally associate with a governmental structure. One can only speculate as to its true nature; although it is doubtful that it was a democracy. Democracy appears to have been a uniquely Athenian feature. The successors to the Dravidians, the Aryan tribesmen of Eurasia, were organized into tribal units each ruled by a Rajah, or chief who governed with the aid of a council of tribal elders. There were apparently hundreds of these local tribal groups, each with its own rajah, but no centralized government existed. Over time, the individual rajahs did attempt to extend their influence over neighboring communities, most often by conquering them. Interestingly, the word rajah, of Indoeuropean origin, is closely related to the Latin rex meaning "king," and the origin of the words "regal" and "royal." Although Indoeuropeans apparently influenced languages throughout the Eurasian continent, their system of government (or lack thereof) did not extend beyond the Indian sub-continent.
After studying the different artefacts, the plan of the cities ,etc. many people proposed their own hypotheses.For example if we consider the cities... they were amazingly well planned with broad main streets and good secondary streets. Each city was laid out on a grid plan with a high citadel and a lower city of domestic dwellings. Urban planning is evident in the neat arrangement of major buildings contained in the citadel, including the placement of a large granary and water tank or bath at right angles to one another. The lower city, which was tightly packed with residential units, was also constructed on a grid pattern consisting of a number of blocks separated by major cross streets. The cities had an elaborate public drainage system. Sanitation was provided through an extensive system of covered drains running through the length of the main streets . Based on these facts Gordon childe proposed his "Municipal administration hypothesis"(Each city is separately administered...cities are not under control of single administration) and A.L.Basham a great australian indologist proposed "Centralized Administration hypotheses"(All cities/towns under control of one administration -- because of similarities in the structure and pattern of the cities/settlements)
Some historians believed that there was priestly rule .Historians who took into account the flourishing trade proposed Merchant rule hypotheses .