Supporters of tariffs on particular products are not significantly more likely than opponents to base their vote for a politician on the
Why this is right
Whenever we see a Necessary Assumption answer is ruling out an idea with the word "not", it's worth negating the answer by removing the 'not' to see if it turns into an objection. If it were the case that "supporters of tariffs are significantly more likely than opponents of tariffs to base their vote on a politician's stance on tariffs", does that weaken? Definitely. That sounds like a way to argue that "even though most voters oppose tariffs, a politician could still be wise to vote for tariffs." For a lot of people, voting is complex calculation of many different factors, and their opposition to tariffs might be really far down the list. (Sort of like choosing a law school is a complex calculation of many different factors. It could be true that "most law school applicants don't want to live in a state that charges high taxes", but see if that keeps them from accepting admission to Stanford Law) Some voters are single-issue voters: they care so much about one issue, that they vote based on that. (Pro-life / anti-abortion is one such issue that is famous for being the reason a voter chooses his/her preferred candidate) So this answer, stated positively, is saying "our author is assuming that tariffs aren't an issue where it's way more likely to HELP you if you support it than HURT you if you oppose it."
Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.