The Worst Spoiler

Here's a question I've been pondering:

Imagine an election with three parties: A, B, and C. Suppose that the election is decided on a plurality-winner (first-past-the-post) basis, and that B and C both would prefer the other to win over A (that is, B is C's second choice and C is B's second choice). Consequently, many members of both parties support "strategic voting"  whereby if B is the best chance of beating A C's voters should vote B, and vice versa.

In two districts, the results of the election are as follows:

District 1: A = 45%, B = 43%, C = 4%

District 2: A = 42%, B = 24%, C = 34%

In both districts, A wins, even though the combined B + C vote is larger.

My question is which party, B or C, is more to blame for failing to "vote strategically".

On the one hand, in District 1, C clearly had no chance of winning. So the voters who did pick C did so, under this view, on a completely selfish and self-destructive basis. But, one could say, the relatively small numbers who voted for C demonstrated C backers, as a whole, probably did try to "do their duty" and vote strategically. Maybe 4% is about the minimum one could plausibly expect even when voters are thinking in strategic terms.

On the other hand, in District 2, B arguably was competitive with C -- even if C had an advantage, B wasn't obviously drawing dead like C was in District 1. It's one thing to say "don't waste your vote on a third party that will be an obvious non-factor", it's another to demand dropping one's first preference in a situation where it is realistically in the mix. Yet of course, the flip side is that in District 2 it looks as if B backers didn't even realistically contemplate voting strategically, and thus a district which is overwhelmingly "B + C" ended up in the "A" column (whereas District 1 at least looks to be on face a swing district).

Obviously, part of the answer depends on what the baseline levels of support would have been for voters had they not been acting strategically. It's hard to credit B backers in District 1 for voting strategically if their base of support was 4% to begin with.

But anyway, this is something I've been pondering.