Former Labour MP Nick Palmer says (via):
AV is great for medium-sized centre parties, since they are normally everyone’s second choice, and their voters often get to choose between the other parties, effectively giving them an extra vote.
It’s also quite good for small parties: they probably won’t win more seats (especially if they’re on the political fringes), but at least their supporters can show their support on the first round before giving others their second preferences. It is correspondingly not so good for big parties, especially if they think that the other big party will get more of the second preferences.
This, a pretty good statement of the standard view of life under AV, is true in the short term. But over time, AV could totally transform our party system.
Say that in the constituency of Trumpton, the Red Party has 55% support and the Blue Party 45%. An easy Red win. But there are disagreements among the Reds, and the party splits: now the Crimson Party gets 25%, the Pink Party 30% and the Blue Party 45%. Disaster: a Blue win!
The Reds, however, can see this danger in advance, and so stick together.
But under AV, they can split with less risk to the overall reddish majority. Almost all the Crimson second preferences will go to the Pinks, giving them a win. In nearby Camberwick Green, the Crimsons come out ahead of the Pinks, and while the Pink second choices spilt a bit more evenly, the Crimsons still win there. Likewise, the Blues, who have majority support in Chigley, can split into the Cyan and Azure parties without letting the Reds in.
Back in reality, established parties would try to stay in one piece; inertia will stop significant splits happening for probably an election or two. But if the Labour leadership seriously antagonised its left, or the Tories their right, then you could easily get a sizeable breakaway group, either setting up on their own or merging into an existing smaller party. You wouldn’t want to rule out the Lib Dems rupturing, either.
So AV could give us a political landscape with five or six parties all polling in the 10-25% range. We would always have hung parliaments, but these wouldn’t be based on one smaller centrist party perpetually deciding which of the two bigger ones it’s going to shack up with this time.