What about strategic voting?

This might then engender a campaign to get all the we-don’t-want-C voters to vote for whomever of A or B is seen as “most likely” able to beat C.  If this works, one supposes, then C will be defeated, and the agreed-upon A or B will win.

Of course all of the we-don’t-want-C voters weren’t going to vote C anyway, but are not likely all to agree upon which of A or B should win or is most likely able to beat C.  There will usually be some rationale put forward for each, such as A’s or B’s (or their respective parties’ previous candidates’) showings in the previous election, or the current published opinion polls, and so-on.

But there’s no real, reliable, way to coordinate this effort.  Some voters will buy-in and vote A instead of their preference B, and other voters will buy-in and vote B instead of their preference A, and other voters will just vote A, or B, regardless, because they don’t buy-in at all.

In the end the net effect will be negligible, one way or the other, though this would be difficult to measure;  it is questionable whether this has ever actually worked.

Another approach is to devise a more formal cooperation strategy at the party or electoral-district level, where, for districts where it “makes sense” to get the A– and B-parties to get together and field only one AB candidate against the C candidate who it is desired to defeat, with the underlying hope that the A-voters and the B-voters will then vote AB, and C is defeated.

Leaving aside the obvious political difficulty in getting “the A– and B-parties to get together and field only one AB candidate” — if this could be done at all, while it might then work for all the voters who would really prefer either A or B over C, such that an AB candidate might be an acceptable compromise to them, there might also be voters who would vote A, but prefer C over B, or who would vote B, but prefer C over A, so that if presented with only an AB candidate, depending on whether they perceive him or her to be really an A candidate or a B candidate, they will end up voting C where they would not have done so had the A and B choices both remained.  There might also be A or B voters who are put-off by being offered only an AB vote rather than their choice of A or B, and will simply not vote at all.

In the end, all else being equal, the votes for the AB candidate can be expected to be less than the sum of the expected A votes plus B votes and less those people who decided to stay home, and the C votes will be more than the original C votes by the number of A or B votes driven to C by the AB candidate.

The AB cooperation approach will to some extent backfire into increased support for C.  Whether this shakes-down to a net benefit for the AB candidate vs the C candidate is moot.  But, despite the noted backfire, if C has only a few percentage points lead at the outset, for this strategy to succeed it only needs to improve the AB percentage by those few percentage points.

All else is not likely equal, however: every election is a new battle — different terrain, generals, and combatants, and different issues, so the outcomes of the previous election don’t necessarily carry forward with any relevance:

“No man ever steps in the same river twice, for it’s not the same river, and he’s not the same man.” — Heraclitus

Provincially in BC such a cooperation approach is pretty much already the case as regards the BC Liberal Party, which is really a de facto coalition of liberals, conservatives, and orphaned former Social Credit members (which was itself such a coalition!).  It’s really a we’re-not-the-NDP-or-Green party.

This coalition continues to exist because it has been electorally successful, albeit with considerable internal stresses due to its philosophically-disparate composition.  But, so far, the things holding it together have been stronger than the things pulling it apart.

But it is an artifact of the FPTP system.  With a preferential-voting system such a pre-election coalition will be unnecessary (though after-election coalitions in the case of a minority government are still in order), and will fracture along its internal fault lines as its membership reverts to their respective true colours:  the BC Liberal party would become a party of, well, liberals, and conservatives would, one would suppose, resuscitate the BC Conservative party, and so forth.

Strategic voting also arises in regard to candidates who are simply not seen as being able to win.

Such perceptions, however they arise, are fed and fanned by the press as well as by published public opinion polls and tend to be self fulfilling.  People want to place their vote where it is most likely to have best effect, so, based on a perception of inviability of a given candidate, they will move their vote to a candidate who perhaps isn’t their first preference, but who they think has a better chance to win.  (Duverger’s Law)

This means that the press and opinion polls have an enhanced ability to influence the outcome.  It also makes it very difficult for low-profile candidates, independents, or candidates from smaller or start-up parties to gain a foothold.

“Plurality voting is simple, and theoretically provides incentives for voters to compromise for centrist candidates rather than throw away their votes on candidates who can’t win.  Opponents to plurality voting point out that voters often vote for the lesser of evils because they heard on the news that those two are the only two with a chance of winning, not necessarily because those two are the two natural compromises.  This gives the media significant election powers.  And if voters do compromise according to the media, the post election counts will prove the media right for next time.  Condorcet runs each candidate against the other head to head, so that voters elect the candidate who would win the most sincere runoffs, instead of the one they thought they had to vote for.” — Condorcet Method