"Suppose party A's policies are bad but party B's policies are twice as bad and the choice is between A and B."
The policy of the real Party A and Party B you're referring to are equally bad, because we end up with the same thing in either case, which is a despotic economic world order that includes constant perpetual war and the profit motive determining the outcome of everything. If Party A and Party B have the same donors, and are both filled with and controlled by multi-billionaires, you're going to get the same thing with each. The "representatives" in each are going to be people cut from the same cloth with the same personal interests, and essentially the same type of character because all were raised within the same bubble. Any "differences" are all going to be empty rhetoric designed to win people over by manipulating their emotions, because in the end both are going to deliver and maintain the same world order.
This is why, for instance, the majority of the Democrats claim they support abortion rights, yet Roe vs Wade can get overturned under *their* watch, and they then insist they can do nothing about it but "hold their positions." That's because one party wants those rights taken away and the other simply doesn't give a shit because these laws do not effect or limit the choices of the wealthy.
"Isn't it rational to vote for A?"
First off, that situation does not exist in the real world, because as I mentioned, those policy differences all amount to little more than buzzword rhetoric.
Secondly, no, it's not rational to vote for any type of evil. The rational move is to build a better alternative, one that actually has *good* policies not controlled or beholden to wealthy donors or run by wealthy politicians. We are capable of doing that because *we the people* have the numbers!
"Your point is surely that you don't see this as being the real situation where the Democrats and the Republicans are concerned, that you think they are equally terrible. You might be right, but you would need to make the decision for 2024 based on the specific candidates who stand then."
We already know who the specific candidates for 2024 will be. Prepare yourself *not* to be surprised but very pissed off. We're getting the same corporate war-mongering pre-selected lot as before, Hillary Clinton, Kamala Harris, Pete Buttegeig, and possibly lying careerist corporate turncoat SJWs and Perpetual Victim AOC. You seriously believe that the DNC will allow an actual progressive, non-corporate candidate to win the primaries? Bernie Sanders turned out to have been an ineffectual, cowardly cuck all along, with no intention of fighting for any of his good policies but simply to "hold the position." That means continuing to say what we the people want to hear, but never actually doing anything substantial to get them passed. Instead, he got numerous progressives on board with the Democrats when he ran for pres under their ticket (he's actually an independent), then when the predictable thing happened, he backed down and told everyone to vote for the corporate choice. And he did this *twice in a row.* It was *inevitable* that Clinton got the primary in 2016 and Biden got it in 2020, and Bernie was intended to help that happen both times.
So yes, it should come as no surprise to you who the DNC is going to allow to run in 2024: another awful, corporate plutocrat who will be dedicated to further antagonizing Russia and China into war, further genocidal attacks in Middle Eastern nations like Yemen and Syria, and the continuation of the system that impoverishes billions of us.
So, no, it's not rational to vote for the Democrats. It's rational to work towards creating an actual alternative that isn't evil, "lesser" or otherwise.