Since the IRS considers Secular Humanism to be a religion, I guess SH organizations can also endorse candidates. But, since just about every atheist I've ever met in real life always votes for the Democratic candidate in all elections, I guess there is no need for that. For the matter, almost all conservative Christians always vote for the asshole party so I guess there is no need for them to endorse anyone either.
My late mother was a rare exception, but she did tell me that her pastor told the congregation back in 2012 not to vote for the "Marxist Muslim" Obama, so yeah, this is nothing new. She voted for Obama because she didn't like anyone telling her how to vote. She also knew that what she was told was bullshit, even if she didn't use that exact wording.