Actually, Bush's (and those of others) objections being largely superstitious is very accurate.
This is because of the nature of the stem cells. Embryonic stem cells are more flexible since adult stem cells are already somewhat specialized -though there is the added advantage of low risk of rejection if the donor is also the recipient. But, more importantly to this point, embryonic stem cells are very plentiful since they can be harvested from the embryos (which are clearly not sentient since there is no formed brain or nervous system) of clinics which perform in vitro fertilization. These embryos, if not harvested for their stem cells are simply destroyed.
So not utilizing them for a good purpose is completely based on the superstition that, somehow, these embryos (often just blastocysts of a few hundred cells) are equivalent to fully formed humans or even a more advanced fetus. They aren't. There is no brain. There is no nervous system. They're cells and it is no more unethical to harvest their stem cells than it is to take a series of antibiotics which obliterate the bacteria in one's body.