You hit the nail on the head by pointing out the voting method. But I think you can pinpoint things a bit deeper with Duverger’s Law.
Duverger’s Law indicates the tendency for a democracy to support multiple parties according to two factors:
- A low enough threshold to get elected
- The psychological ability to support candidates without fear of wasting a vote
It’s true that Duverger said proportional representation (PR) methods tend towards multiple parties. But in his book Political Parties he explains the reason why PR tends to multiple parties. It’s because of those two factors.
PR does especially well with Duverger’s Law because it passes both of Duverger’s factors. One, a PR method has a lower threshold, which can be estimated with the Droop Formula for a multi-member PR district or set with a party-list method when electing a larger body at-large.
Instant runoff voting (IRV) is not going to encourage multiple parties because it passes neither of the two factors. First, it’s a single-winner method so it has a high threshold. Fail on factor one. On the second factor, voters are still punished for ranking their favorite as first. Like plurality voting, voters can still be better off by ranking a compromise candidate as first. This short video explains how. Fail on factor two.
On (traditional single-winner) approval and score voting, both fail factor one. This is just inherent to being a single-winner voting method. They have multi-winner proportional counterparts, but going into that would be talking about a different voting method just as talking single transferable vote (STV) is different than talking about IRV.
On factor two, both approval and score voting come through. You can always support your honest favorite and give them your maximum support with either of these voting methods.
Given that score and approval voting only pass one of the factors, it’s unlikely that we would see nearly as much proliferation of new parties as we would otherwise see with a proportional method. But they could get competitive wherever there was an opportunity — particularly moderate candidates and parties, which approval and score tend to favor.
Probably the biggest boost to third parties with approval and score voting will be how simply third-party support registers. It’s easy to see that support, unlike the results from an IRV election — the algorithm hides much of the ranking data at any one point in time. The clear reflection of support with approval and score voting makes it easier for third parties to grow with voters now being able to throw in their vote without regret. Minus proportional representation, it’s the best gift a third party could get.