They’re not defending the practice as Republicans are envisioning it.
What they are defending is the sometimes-necessary practice of late-term abortions when the life of the mother is at risk.
Yes, Bernie and other Democrats could simply say, “I don’t support ‘birthday abortions.’ That goes without saying, as such things are already illegal, and do not happen.”
But I guarantee if they did that, the Republican response would be: “But sometimes late-term abortions do happen!”
You make the mistake of assuming that Republicans are arguing in good faith. They don’t do that.
The fact that Republicans are saying that when Democrats voice support for late-term abortions to save the mother’s life, they are actually supporting “the legalization of infanticide”? Well, that’s all the evidence you need that they aren’t arguing in good faith.