Just started the book. On page 9, he describes how someone would gain entry into the Senate: every year 20 Senators “were elected to replace those who had died in the previous year or had become too poor to keep their seats.” Cicero “tops the poll” and becomes Senator, with official title of quaestor.
This all feels very different from what I’ve learned. I was under the impression the censers and magistrates CHOSE Senators based on their financial and social qualifications. I also know that once someone was elected as a magistrate, then they automatically became a Senator... maybe that’s where his confusion comes from?
Does Harris have this as wrong as it feels like? How accurate is the book on the whole? I feel like if he’s not going to get details like Senate membership correct, may not be worth reading.
I think, although /u/Alkibiades415 summarized Lintott well, he neglected to address your specific understanding of how the Senate was formed and how Harris describes it.
No one elected senators. The Senate was a body (until Sulla) formed of former magistrates. The elections were for magistrates, who became senators when they left office. The magistrates were elected regardless of how many senators had died during the year in question; they were elected in numbers according to how many magistracies were vacant - which would be as many as there were, since Romans elected magistrates yearly. That number increased over the life of the Republic, and the magistracies that qualified for membership in the Senate increased (as /u/Alkibiades415 indicates), but nevertheless elections were for magistracies, not for membership in the Senate. If Harris indicated senators were elected, he's wrong.
Flower has an interesting point about Sulla in her book Roman Republics. Sulla, in his dictatorship, transferred the courts from the Equites to the Senate, so when each court needed some 50 jurors there was need for more than 300 senators to staff the juries. This in part accounts for the rise in numbers of quaestors and their inclusion as later senators, but Sulla also appointed at least 300 senators without election. That, as Flower says, is a drastic change of the nature of the senate, both in function and in sourcing. Half the senate if not more now had no experience as magistrates and no voting constituency. Caesar also appointed senators, and Augustus, so over the course of the Late Republic the link between election to a magistracy and membership in the Senate became increasingly distant.
So, censors did control the lists of the senate (a point I think you grasp and so I haven't discussed); magistrates didn't choose senators and Romans didn't elect senators. Romans elected magistrates. On leaving office, magistrates became senators. Until Sulla changed everything.
Until the second Punic war, the rules for admission to the senate are a little hazy, but certainly the main gatekeepers were the censors. It seems that admission was owed not so much to your social rank (ordo) or wealth, but rather to your personal accomplishments (usually by progressing through the offices of the Republican government). During the second Punic war, when many senators were killed and could not be replaced, things began to change slightly. First, rank (ordo) became more important, rather than personal accomplishment. Presumably, this means that some men who were of sufficient wealth could be admitted to the senate without holding a major political office, if the censor(s) wished it. The list of offices which auto-qualified one for the senate also began to expand. Aediles, first just the "curule" aedile but later both of them, became qualified for senatorial duty. Around the end of the 2nd century BCE, tribunes of the plebs also auto-qualifiers (before they could summon the senate, could enact veto in it, but were not necessarily apart of it after their term). It was in 81 BCE that Sulla extended the privilege (or requirement, depending on how you look at it) to the elected quaestors. This was Cicero's "in". His family was rich and well-placed both in their local town of Arpinum and also at Rome, but the barrier to entry into Rome's elite class was very high in the 1st century BCE. Cicero set himself apart from the many many other rich young equestrians by being very smart, very savvy, and very well-spoken in public. He was lucky (or skilled) enough to get Q. Mucius Scaevola as his tutor in Rome, an essential early connection which got him moving in the right circles. In other words: even getting elected quaestor was no easy task, and you had to do a lot of legwork before that to even get a chance to be elected.
One ancillary note: the "official" number of senators before Sulla's dictatorship was about 300, but that seemed to be a general guideline. About 150 was probably the norm at any given meeting. We find instances of a quorum of 100 in the 2nd century (the minimum number required to have a meeting). Putting aside those who had been elected to aedile and above, there were probably others who were enrolled in the senate because of their birth, or their familial connections: patricians/nobiles who were too young to be aedile, or had decided not to pursue politics. Those participating in priestly jobs were certainly also included. There were probably honorary members, too, like aristocrats who had suffered debilitating war wounds, had health problems, or disabilities from birth.
The personal "requirements" to be a senator are nebulous before the time of Augustus. There was no fixed property requirement, but there seems to be a general understanding that you had to have your affairs in order and enjoy the leisure time afforded by a certain amount of wealth. You had to be present in the capital year-round, which carried a certain price tag with it. You also had to have a good character, whatever that means. We find plenty of senators in the first century with apparently odious characters. Like all the rest, the decision was ultimately up to the censors, and one could be added to or struck from the record at their whim.
For more, see Lintott, The Constitution of the Roman Republic.