"Some prominent citizen from a useful tribe? A sabatini, perhaps? A pomptini?"

Rome was split into thirty-five ancestral tribes, into which Roman citizens were separated for voting and for the legislative assemblies. The sabatini and pomptini were two of these tribes. Cicero is hoping for clients from as many different tribes as possible in order to spread his support. Some tribes were viewed as more useful than others; those composed largely of rural citizens might not hold much weight if the men could not spare time to travel to Rome to vote.