As Person B's reply to Person A's comment is a possible (and viable) explanation for the polling trend, and does not impugn Person A, I don't see that Person B's response is
ad hominem. Person A's reply to Person B's comment is, however, literally a
tu quoque response.
@Rich McMahon:
Person ‘B’ observes/ concludes from this polls’ change that Political Party priorities (presumably) outweigh other factors that should benefit our Country i.e., ‘decency”. Note: this is important, as Person ‘B’ is stating an opinion (only).
This is not a matter of opinion only-- though Person B fails to cite any evidence. There is plenty of evidence confirming this, provided by Alabama voters themselves. The GOP's decision to fund Moore's campaign and Republican walking-back of previous criticism of Moore after Trump endorsed him (both subsequent to the posting of the question) are further evidence. Even a Republican noticed; Jeff Flake (R-AZ)
responded to this evidence by writing a $100 check to Doug Jones's campaign, with "Country over Party" in the memo.
Opinion or not, Person B's response to Person A is germane to Person A's comment, as it it on topic and offers a possible explanation of the polling trend cited by Person A apparently with no additional comment. It is arguable--
and I would love to have seen Person A offer an explanation other than the one offered by Person B-- but Person A deflected and resorted to dragging out a red herring instead of responding to the topic at hand.