Practically, could an employer prevent an employee from voting?
As was noted in the comments, I believe the answer here is yes it is possible. I think from the vast diversity of employers out there and, judging from the fact that many employers urge (coerce?) their employees to vote a certain way certainly implies that the will to affect outcomes of elections is there for employers. However, the possible downsides of doing this to an employee (which include a devout pummeling in the court of public opinion) would no doubt deter all but the most ardent employers from actively preventing employees from voting which, as noted by others, would be minimally effective at best.
That being said and this being the US, we know it has already happened. I stumbled across this series of letters which describe a sawmill owner's efforts to disenfranchize both black and white employees of his from voting for the "radical ticket" in South Carolina, 1868.