Much like the Original U.S. Senate the Roman Senate was not directly elected. They were a body of 600 magistrates or ex-magistrates elected minimally to the office of Questor and appointed by the Consul to the Senate for life when there was a vacancy.
So to be a Senator one had to win at least one elected office (Questor at least), and be appointed by another elected official (a Consul).
Much like the Original U.S. Senate the Roman Senate was not directly elected. They were a body of 600 magistrates or ex-magistrates elected minimally to the office of Questor and appointed by the Consul to the Senate for life when there was a vacancy. So to be a Senator one had to win at least one elected office (Questor at least), and be appointed by another elected official (a Consul).
The Roman Senate was made up of appointees and volunteers from old families, all of whom nonetheless were subject to the Senate's own pecking order, along with former higher officials such as ex-Consuls. Senators mostly remained there for the rest of their lives, with no fixed term, and no standing for reelection, not even accountable to those responsible for their senatorial status.
Prior to the early 20th century US Senators were selected by state legislatures. The terms were six years (just as now). Rome had no state legislatures as such, but occasionally recognized expedient arrangements (such as tolerating Armenia as a buffer state).
Consuls were elected by an assembly of the male citizens of Rome, and while a large body, it wasn't itself either representative or elected. The power of consul was limited by mutual veto power (for a while now I've wondered if this weird dual ruler arrangement was in emulation of the Spartan dual kingship) and the term limited to one year. Also, various limits were imposed on it (no succeeding oneself, minimum age requirement).
Sulla restructured the popular assemblies in such a way that the Senate and the aristocrats had an even tighter grip on power. Pompey and Crassus overturned those changes while joint consuls.
*. Caesar's "tyranny" included expansion of the Senate to include members of newly provinced and other conquered lands. Pompey wanted land for his numerous veterans of his major successful military campaigns. Caesar wanted the same thing for his own veterans. The Senators had been inheriting as well as taking over Italian farms etc, most significantly from those serving in Roman armies, a problem dating back to at least the time of the Gracchi.
His murder in the Senate was very obviously not the act of supporters of republicanism. Clearly they were Democrats.
BTW, I'm enjoyin' this. :')