[EM] Challenge: two-party methods

[Juho Laatu]

After some recent discussions and thoughts around two-party systems I thought it would be interesting to discuss two-party systems also in a more positive spirit. The assumption is thus that we want the system to be two-party oriented. We want to have two strong parties, and one of them should rule. We want to allow only well established parties with wide support to rule. The first obvious approach is to ban all other parties than the two leading parties. But maybe we don't want  to be so brutal. Let's not ban the possibly already existing, much liked and hopeful third parties. It is also good to have some competition in the system. Let's not allow the two leading parties think that they don't have to care about the voters and they can do whatever they want, and stay in power forever.

What would be a good such method? In addition to what was already said we surely want e.g. to avoid the classical spoiler problems.

The target is roughly to define a "good" method that
1) elects either one of (or one of) the candidates of the two leading parties
2) but allows small parties to become leading parties one day

Is this a sensible approach to good two-party methods? What other approaches or criteria could there be? The target above thus assumes that we don't want to elect from small "third parties", but we may elect their candidate as soon as they can beat one of the two leading parties in popularity (and become "second or first parties"). The key difference to general single-winner methods is the target not to elect ("small") compromise candidates between the two large parties but to elect rather from one of those large parties.

Heres one draft of a method for you to consider.
- let's assume that each party has only one candidate (to keep things simple for a while)
- voters will rank the candidates
- use some proportional method (e.g. CPO-STV) to pick two candidates (=parties)
- use Plurality to elect either of those two candidates (using the same votes)

The behaviour of this method resembles IRV a bit, in the sense that it favours large parties and in some sense avoids "weak candidates" and requires "core support". But maybe this algorithm makes a bit more sense, e.g. avoids too early elimination of strong candidates with little first place but lots of second place support.

Juho


P.S. Could there be also three-party or n-party systems? Limiting the number of parties to n would be an alternative to thresholds. This approach could be used also in a two-party system, i.e. set the threshold e.g. to 33% (or lower, or higher). Does the proposed method work better than such thresholds or simply picking two largest parties?