I think this idea is absolutely the right thing to do, and this is absolutely the right time to do it.
The idea that this will stop “the best candidate” is a straw man. For any job, there is no perfect candidate, but a set of criteria, and a number of candidates who match this criteria to a greater or lesser extent. All this does is add an extra level of criteria in the essential catagory. In politics we all know that there have always been other factors than simply “merit” in appointing people, and that this is true in all parties. Factionalism is far more damaging to politics than gender balancing but is allowed to carry on regardless as it’s an informal rather than a positive and formal rebalancing.
I hope this wouldn’t be a permanent measure, but one designed to permanently change the culture until it makes itself unnecessary – like All Women Shortlists.
Politically, I don’t think this can hurts us. Anyone unlikely to vote for a party based on this issue alone, is always going to be unlikely to vote Labour. On the other hand, this single measure gives us back a sense of radicalism and transformative politics that has been missing from the Labour Party for a while. It could have the power to further inspire the base, particularly the women, and to bring in a generation of feminists who see Labour taking real and direct action to improve our offer to women in politics.