I feel like there is nothing wrong with "sleepovers" in reference to voting. In the not so distant past, some people had to travel great distances just to participate in voting. If an individual wants to beat the lines and having to wait to cast their vote, I see nothing wrong with that.
The practice of sleeping over in order to cast a vote is appropriate and is not a new occurrence. In the not to distant past people needed to travel great distances in order to cast their vote and it was totally acceptable for large numbers of people to camp out days in order to cast their vote.