I was surprised to learn that this is not illegal. Sorta.
Setting aside class protections which may exists in a few scattered states, Federal Law 18 USC § 594 reads:
"Whoever intimidates, threatens, coerces, or attempts to intimidate, threaten, or coerce, any other person for the purpose of interfering with the right of such other person to vote or to vote as he may choose, or of causing such other person to vote for, or not to vote for, any candidate for the office of President, Vice President, Presidential elector, Member of the Senate, Member of the House of Representatives, Delegate from the District of Columbia, or Resident Commissioner, at any election held solely or in part for the purpose of electing such candidate, shall be fined under this title or imprisoned not more than one year, or both."
Since the law makes it illegal to influence a vote, which necessarily must happen before the voting date occurs, my reading of the law is that firing a person for voting a particular way, or failing to vote a particular way, would not be illegal.
Intimating one would or might fire an employee for voting for a particular candidate, or failing to vote for a particular candidate, regardless of whether or not the threat was consummated, would be illegal.
This is important because the burden of proof is not upon the employee to prove whether the firing was related to the vote, which is obviously difficult to prove. Rather, it hinges upon the threat before the fact, which might be difficult to prove but is an action on the part of the employer as opposed to a state of mind of that employer.
Whether or not a statement like "If Obama is elected I will have to let some people go" is vague enough to get away with, I don't know, but the sheer number of conservatives who are full-on giddy at the attempt to tamper with democracy is disturbing. This is not a handfull of wingnuts, there are literally thousands of Republicans in ONE thread on ONE facebook page not only defending the tactic but encouraging more of it.
I find it difficult to imagine something more central to the American ethos than the right to vote, unmolested, nor would I be engaging in demagoguery by suggesting those who would champion such a tactic be summarily rounded up and deposited just past the border.
Having found ours lacking, perhaps they can coerce each other into a more perfect union.