Why would anyone on a science site bother to mention that a scientist has endorsed a Democrat?  It's only news when a scientist does not endorse a Democrat - well, if anyone in science media were to report it.

In 2008 76 Nobel laureates endorsed Senator Obama and, of course, blasted Bush. What has Obama done differently?  Not much, he simply replaced conservative anti-science positions with progressive ones.  Did he have science reports edited to match his ideology? Check. Do we still limit federal funding for hESC research?  Check.  Subsidize the president's pet energy projects?  Check.

So 76 brilliant people endorsed Obama in 2008 and in 2012 we have to feel sorry for the guy - if he wins, he is going to inherit a real mess. 

Now, most scientists are liberals.  Before conservatives get up in arms about that, let me clarify that they are the good kind of liberals - the kind necessary for science and the kind that make America a liberal democracy. Great science is not done by being conservative. A few are creepy, social authoritarian progressives, and they are the loudest and most annoying, but most scientists don't like them either, they just don't want the drama so they endure them. Yet it might be good for scientists to speak up for science, not just politics, more often. I have often argued that scientists should at least pretend to not be so partisan.  The reason is simple; if Democrats know you are in the bag for them, they will ignore you. Police and teacher unions are in the bag for Democrats, for example, but individual members are harder to corral so Democrats have to cater to them. Scientists have no union, yet they are willingly letting it be known they vote Democrat 6:1 and for that reason Democrats don't have to do anything for them and Republicans know it is pointless to try.  When has science funding declined in the last 40 years?  Only when Democrats were in the Oval Office. Funding is what matters, not fairy tales about how much one party loves science because only 30% of them deny evolution yet the other side is anti-science because 39% of them deny evolution. And Republicans love science.

Cameron English, writing at PolicyMic, says
If he wants to promote science literacy, Nye should be doing everything he can to disassociate it from both political parties, because neither deserves to be associated with science. 
Well, that's true.  Both have their anti-science contingents.  It's unfortunate for the credibility of science media that the public only gets one side of it, though.  I get that the most vocal people are going to be young and earnest, and that includes in science, but it's bad for all of science if Republicans believe that science is partisan.
Bill Nye's goal to get more kids pursuing careers in science is admirable. And I think just about everybody agrees that more people knowing more about science is desirable. But endorsing politicians is no way to get more Americans interested in science. 
Right, it knocks Nye out of the issue for 50% of the public - which means he is only talking to the echo chamber already voting for Obama.  He is doing nothing at all for science, he is just engaged in a partisan get-out-the-vote campaign.