I would claim that there are two factors that lead to the fact that most scientists are Democrats: the overall lean of universities, and the source of funding.

First, scientists tend to work in academia. It's well documented that universities are strongly slanted to the left. It seems reasonable to expect that when the left-leaning universities are looking to hire, they will tend to (possibly not intentionally) favor others like themselves.

Second, the government is the largest funder of scientific research. However, to the extent that the GOP favors a smaller government (and though they say this, they don't actually do so, but that's a different discussion), this would make one expect that those against such governmental funding would be morally bound to find other means of employment.

asking how the guy who prohibited stem-cell research

This isn't what happened. It's incredibly frustrating to me that people continually read a ban on federal funding to be a ban on research itself. But it does reinforce my point that the governmental force behind science is now perceived as the sole force behind science.

reinforce the idea that Democrats are the party of science and rationality

In the science of economics the opposite is true. For example, Democratic support of minimum wages and other price controls, and especially their opposition to free trade, fly against the wind of economic science.