Most pro-lifers, and virtually all pro-life Democrats, are not single issue people.  This means that they are not leading the battle on pro-life issues, but that they can be counted on to collaborate with the single-issue pro-life leaders on some of them.  The single issue people need them, so they have a bargaining position.  

This is no different than the pro-choice side of the debate.  Most pro-choice people, and virtually all pro-choice Republicans, are also not single issue people.   But pro-choice folks can count on them in some key battles.

The single issue people on both sides gain power by being able to threaten their friendly party by endorsing friendly supporters of their issue in the hostile party.  This keeps their issue at the top of the agenda of the friendly party, even as it works against the friendly party's immediate interests in winning elections.  It's smart politics whether NARAL does it or National Right to Life.  But it makes life for us multi-issues partisans much more difficult.  It's supposed to, and it's just as valid a part of democracy as being a partisan Democrat is.