I think his threatening a lawsuit about his stance on abortion was just a shot across the bow as a "warning" to other candidates...about the real issue Trump is afraid they'll bring up. A hope he has about keeping them too scared to talk about it. And that is, how Trump says the gay marriage issue is a "dead issue" and how of the several gay weddings he attended, he called them "beautiful" (on youtube).
If his followers found out he considers gay marriage "beautiful", that would be it. Abortion, people in his bloc are all over the place...sure it might cost him a few votes but not knock him that seriously. If folks in his bloc found out about his adoration for gay marriages....they're pretty much unified just right of center on that Decision last year that stripped the right to vote on the matter out of 50 states' hands...complete with a rainbow light show on the Whitehouse that same evening (when it takes days to get approval of any changes at the Whitehouse..)...
Abortion isn't bothering Donald as much as his position on gay marriage. And he means to stop it in its tracks using a surrogate issue first as a "warning".. Ie: he's showing his weak scales by the ferocity with which he defends them..