[Someone]

There is no guarantee that the effect of an almost certain victory/loss is identical for all parties. On the contrary, there is some data that indicates that supporters of some parties are more motivated to vote. See for example http://www.ncbi.nlm.nih.gov/pubmed/22065127, which shows that weather conditions can affect election results. I cannot find data on it, but I think it is not inconceivable this extends to 'going to the polling station even if it does not make a difference to the result'

However, in a winner takes all election, the effect would have to be huge. Let's say all polls indicate a 51%-49% result. Then, at least 4% of the winning party's voters would have to stay home 'because they already won' to change the result (and that assumes none of the other voters stay home 'because they already lost'). At a more realistic 60%-40% poll prediction, one in three voters would have to stay home.