Yes they can affect the outcome, but the opposite direction than you described. People who feel their candidate is doomed to lose don't turn out. People like to

Yes they can affect the outcome, but the opposite direction than you described. People who feel their candidate is doomed to lose don't turn out. People like to vote for a winner. edit: To be clear, when polls overwhelmingly suggest a landslide, it suppresses votes from both sides. But a much higher proportion of the losing side will choose not to vote, thus inflating the gap.