Never (not a left-winger), and doing so should be a felony.
Assuming that this is a special rule and not just part of a wider abortion ban; how would this be implemented? Say that a person decided to have an abortion independently of any Downs diagnosis which comes through after they've made the decision: would you criminalise them? To do so would seem to be wrong in my eyes; and to me is a big reason why you can't start criminalising based on things like this. It also would likely create a situation that is nasty for the child who's mother has made a decision that she doesn't want to look after the kid: and that either means that the kid has to go through a care system that's very hostile to children generally but especially children with special needs; or it'll grow up in a family environment that may well not be overly friendly - especially when you add in the fact that children with syndromes like Downs require lots of hard work to bring up correctly and many people, I'd include myself in this, aren't suitable for the job. To not do so would also seem to be against the spirit of what you support as well though... Plus it begs the question that if Downs gets this protected class: what other disabilities should be similarly protected?
That's why I think that there's a very big difference on this issue between the personal question of whether you would do it and the legal question of whether or not it should be illegal. I think that the former is a complex issue that none of us will know what we'd do until it happens to us - DC Al Fine brings up exactly the same point from a totally different perspective and its one that I think is very interesting - and the legal side of things which I think is slightly more clear cut in that when you start to ban people from having abortions for particular reasons you run the risk of criminalising people who've made the decision entirely separate of that diagnosis. Its certainly not something that I'd seek to ban nor do I think that its possible to ban without adversely harming some women unfairly.