I've read about the process of Roman citizens gathering in centuries based on tribes or locations within the city in order to organise how elections were held - but repeatedly I encounter comments about how this voting structure benefited the upper class' choice of candidate over those from lower class centuries.
I don't understand how this worked in practice. Each century takes its own vote, and then the winning candidate in that century translates to a single 'century vote' for that candidate - and the upper classes voted first within those centuries.
This doesn't effectively explain any advantage to me... Surely even with a preferential place in the century position, or century structure, the upper classes would still be too small in number compared to everyone else to be able to significantly sway the decision?
You're not thinking about this quite right. The comitia centuriata did not divide the centuries up evenly. The first centuries had far fewer members than the lower centuries, but still accounted for exactly the same weight at the vote. /u/LegalAction made up a nice little diagram (which I hope he doesn't mind my using) of the distribution of the centuries. The distributions changed over time, but the general trend of more centuries in the first class and the cavalry always remained. As is readily obvious, the first class alone has nearly as many centuries as all the other classes put together, despite being as a class no larger than any of the others, and in fact probably quite a lot smaller. What's not immediately obvious is that the 18 centuries of the cavalry amounted for a ludicrously tiny portion of the population. Take a look at the lowest century, amusingly labelled "everyone else." That's the capite censi, and amounts for a huge portion of the population, including nearly the entirety of the urban population. All those people have exactly one vote, compared to the 18 votes of a tiny portion of extremely wealthy individuals and the 90 votes of a likewise small portion of people not significantly less wealthy than the cavalry. In fact, the capite censi for all intents and purposes generally didn't have a vote at all--Roman electoral practice closed off the ballots once a majority of the centuries was reached, so the capite censi and other lower centuries frequently didn't get to vote--this then raises the oft-discussed question of how frequently voters in these lower centuries even bothered showing up to centuriate votes that they knew they had almost no chance of being useful in.
The comitia tributa didn't have the problem of unequal distribution based directly on wealth, but likewise had a similar issue of unequal distribution of votes. There were by the late Republic 35 tribes, each of which had one vote, like the centuries in the centuriate assembly. These were not evenly split up according to population, however--at more than a million inhabitants, hundreds of thousands of whom were citizens, the city of Rome should have, theoretically, dominated a population-based system. Instead, of the 35 tribes fully 31 of them were rural, and only four were urban. The distribution of the tribes also did not necessarily follow where people were living at the time of the vote--the population of the city in particular was quite migratory. Property owners were generally enrolled in the rural tribes due to their landholdings in the country, and generally speaking the members of the rural tribes that were able to come into the city during votes can be expected to be those with property, who could easily afford to be off their land during the journey to the city. Additionally, all freedmen were enrolled in the four urban tribes, regardless of location or wealth. There were a few attempts to change this, but they met with serious opposition (Sulpicius died for his attempt, Sulla probably shut down Carbo's legislation attempting the same reform, Gaius Cornelius failed even to allow freedmen to be distributed in the tribes of their patrons in the 60s, and Manilius withdrew his bill in 66 after raising senatorial ire--Asconius also explains a reference in Cicero to a promise that Clodius made during the praetorian elections of 52 to a proposed bill that would distribute freedmen in all the tribes). The distribution of freedmen into only the urban tribes is not insignificant--freedmen made up a seriously big chunk of the population, and if distributed widely across all the tribes were feared to be able to dominate those tribal votes. Their restriction to the urban tribes is therefore decidedly unfair
This is referring to the Centuriata/Comitia Centuriata, or Centuriate Assembly (Army Assembly), a legislative body that was traditionally attributed to the Servian Reforms of the 6th century BC, which also re-organised the army according to wealth classes (dating them precisely is difficult if at all possible). There were other assemblies, but the Centuriata controlled the elections for consuls, praetors and censors as well as much legislation, though after 218 BC these legislative functions (aside from declaring war) mostly fell under the aegis of the Tributa Plebis/Concilium Plebis (Plebian Assembly), for laws proposed by tribunes or the Comitia Tributa Populi (Tribal Assembly) for other legislation. Capital charges were also voted under the Centuriate Assembly until the Late Republic. Essentially, from its genesis in the 6th century BC until the late 3rd century BC and into the Late Republic, the Centuriate Assembly was the most important voting body in the Republic.
It was divided into 193 'centuries' of widely differing size, but each represented one vote. Voting always proceeded from the highest ranked centuries, the 18 equites centuries, and then the 170 centuries of the pedites (infantry) and 5 pointless 'unarmed' centuries. Once a majority was reached, voting stopped, so obviously the lower you were in this order, which was based mostly on wealth, the less chance you would get to have a say.
The reason it was so loaded in favour of the wealthy is that under the Servian Constitution, these centuries were allocated to wealth 'classes', with the equites forming the aforementioned highest class of 18 centuries and there being 5 classes below that, with the capites censi representing the worthless unarmed classes. This aligned with the supposed structure of the army in the Servian reforms, where equites served as cavalry and the First and Second classes served as fully equipped hoplites due to being able to afford more equipment, the Third and Fourth classes having less equipment, and the Fifth class being skirmishers. You can see vague echoes of this system in the later system of velites, hastati, principes and triarii.
The key is that while each century represented 1 vote, each wealth class did not have an equal number of centuries. The equites had 18, and the First Class (the next richest men with a value of at least 100,000 asses) had 80 centuries (40 of iuniores or young men and 40 of seniores or older men). The Second through to the Fourth Classes only had 20 centuries each, 10 of juniors and 10 of seniors. The Fifth Class had 30. Considering that voting stopped once a simple majority (97 votes) was reached, you can see that the equites and the First Class possessed a monopoly on the majority of the votes. As these men, similar in wealth and interests, tended to vote as a bloc, the lower classes would generally not even get to vote and would therefore have no say in most elections and much legislation. The system was therefore deliberately constructed so that "the better off voters would always determine the business, unless they were deeply divided amongst themselves" (North 262).
Source: John A. North, "The Constitution of the Roman Republic," from A Companion to the Roman Republic, Eds. Nathan Rosenstein and Robert Morstein-Marx, 2010, 256-277.
