Two emails, the first is from a friend who works on these issues closely:

Dear Jonah:
I couldn’t agree more about your stem cell point. Embryonic stem cell
research doesn’t involve abortion per se, of course, since it involves
embryos before implantation in the womb, but the basic point that an
important element of the enthusiasm for it on the left has to do with the
fact that it puts the pro-lifers on the side of the dark ages seems right to
me. In the abortion debate, the pro-life forces can speak about love and
compassion and equality, while the pro-choice folks have to appeal to
abstract notions of freedom or else concoct imaginary oppressions or newly
minted rights. In the stem cell debate, on the other hand, the pro-life
forces are stuck defending the humanity of a microscopic organism that looks
nothing like anyone, while the pro-choice (now dubbed “pro-research”) forces
can speak about compassion and help for the suffering, and the like. Of
course, the basic issue is the same: the defense of the defenseless against
the selfishness of the strong; but the emotional calculus (on which,
frankly, so much rests in both cases) is very different.
Watching the Democratic convention this summer, I found myself wondering why
speaker after speaker brought up the stem cell issue, and why the audience
seemed so energized by it. There were many reasons, I’m sure, but one key
one must have been that the issue offers a way for pro-abortion types to
stake out what feels like a kind of moral high ground, and to feel like the
expansion of license is akin to the progress of science.

and from a reader…

Jonah,
I’m on board with you as far as the non-embryonic research is concerned. However, the pro-life crowd has hijacked the stem cell debate much as the pro-choice crowd has. Naturally, abortion proponents don’t want to see embryonic stem cell research banned because that would be tantamount to assigning some intrinsic value to the embryo. Taken to its conclusion, this could mean that they openly root for success in embryonic research and failure in other non-controversial types of research.
The pro-life people want to see the cord blood/adult stem cell route succeed so they can credibly add “Because embryonic research doesn’t work as well, anyway” to their list of reasons for wanting it banned. Whether embryonic stem cell research works or doesn’t work has little to do with the pro-life argument for wanting it banned. However, if it can be proven to be inferior to the cord blood alternative, the “miracle cure” red herring will be squashed and the ban will be easier to get passed.