It's not a baby until it's born. It's still a fetus even if it would have been viable if born. Any other deadline is going to be either arbitrary and not a good fit for some subset of cases. Also, wanting something to be legal is not equitable with wanting something to happen, and you should avoid falling prey to that false logic. I don't think the person you are replying to is saying they would celebrate a last minute abortion, just that they think it shouldn't be something that the blind hand of the legal system is applied to
They will. When primaries come, the Democratic party will soundly and democratically (little d) reject his attempt to represent them by not voting for him. Even before then, he will be rejected because nobody is going to give him the time of day when it comes to setting up a debate or using party resources to facilitate his campaign. Until then, he is not a representative of the party, just a dude who says he wants to represent the party. I could just as easily say I want to represent the Republican party by running a campaign on aborting every single fetus in the United States and then holding a giant gay orgy before forcing all cisgender people to undergo surgical transition. Obviously I would not be representative of the Republican party. Same with this guy and the Democratic party. The only reason it feels like this guy is in any way legitimate or affiliated with the Democratic party is because of how early primary/election coverage starts.
But the deniers mentioned in the article are in the UK, hence the relevance