[jfengel]

Single-party voting is not necessarily irrational. A vote is a compromise. In a democracy, you need a majority to achieve anything, and with many options available, it's going to be rare for any single option to pass 50%. You get things done by making bargains with other voters. And those bargains have to be durable over time: I'll vote for your thing today if you'll vote for my thing tomorrow.

A party is a semi-formalized way of acknowledging those long-term bargains. They are sets of people with overlapping concerns -- sometimes distantly overlapping, where A and B have things in common, as do B and C, but A and C barely recognize each other. But if you can't get A, B, and C all to work together, none of them get what they want.

And in the worst case for them, X, Y, and Z achieve what they want instead -- positions that A, B, and C all agree are bad. The party that behaves with unity will get things that some of its members want and all members can live with. The party that has people opt out every time they disagree will achieve less-than-nothing.

The point is that a party can be a rational long-term decision even if it seems in conflict with short-term interests.

[pier25]

> a party can be a rational long-term decision

Only assuming that there is something like a constant ideology that party members adhere to over the decades.

[jfengel]

Decades is too much to ask, but one decade isn't. It isn't a single ideology, but a set of overlapping ideologies. Those do shift, as circumstances and priorities change, but generally not radically within the scale of a single Presidential term.

I'd say it's not unreasonable to commit to a party and re-evaluate that commitment every decade (or perhaps 8 years is a better figure in the US). That's not to say that you won't be constantly looking out for new information, like a technological change that makes you reconsider your ideas or the discovery that some candidate is very untrustworthy. But if you want anything nontrivial done, it's not going to happen on a time frame shorter than a half-decade -- and your allies are going to want to believe that you'll still be with them once they've achieved your top priority.

[pier25]

> I'd say it's not unreasonable to commit to a party and re-evaluate that commitment every decade

But that's a rational attitude and not what I was describing initially.

A lot of people commit to a party for all their voting life which generally is quite a few decades. Sometimes this even goes for generations.

[jfengel]

Oh, definitely. Parties change, and people's priorities change, and that gradually leads to a strain. Even purely rational people have a hard time telling when that strain has become a breach.

And a new alliance isn't easy to construct. You can't just go to the opposite party and say, "I've swung to you, so start catering to my priorities". Usually, you'll spend some time voting for nobody, in hopes that somebody will notice you, but that's hard without some level of organization, and that's work. In the meantime, you're failing to vote for a party that you had at least some sympathy for, and possibly losing to a party that has no reason to be on your side.

So a lot of people make party-line decisions based on inertia. They could improve their situation, but it's not as simple as changing their vote, since the existing alliances have priorities that won't change for a few votes. That leaves a lot of people in limbo, where there is no really great rational choice.