The population tiebreaker is based on State population of the most recent census at the time the Senators took office. So Tester won't overtake Whitehouse, as the 2000 Census results haven't changed. If there's any overtaking, it'll be if Alabama unexpectedly passes Tennessee and they redo the tiebreaker between Tuberville and Haggerty once the 2020 Census results are finally released.
Is this even possible, since the 2020 Census wasn't formally completed by the time they were sworn-in on Jan. 3rd?
Depends on how they interpret the rule and how the rule is worded. However, the estimates for their States' populations would have to both be radically wrong with Alabama's population much higher and Tennessee's much lower than expected for it to go beyond the theoretical. I strongly doubt that the estimates would change enough for Tennessee to lose a House seat and Alabama to gain a House seat, even with Tennessee still having more people, let alone with Alabama having more. I brought it up mainly because of the new incoming Senators, those two are the only ones whose relative seniority depends upon the State populations tiebreaker.
Weirdly enough, Raffensperger might be able to influence whether Ossoff or Warnock will be Georgia's senior Senator. If he certifies them at the same time, Ossoff will have seniority based on the alphabetical tiebreaker, but if he certifies Warnock first and holds off on Ossoff for a day or more for some reason, such as a recount, then Warnock will be the senior Senator.
