No one likes to fell like their on the outside, and not one wants to back the losing side. Poll results that are released days and hours, even the day, before election day, people will gravitate towards supporting the candidate that is the current poll leader. Voters won't want to "waste their vote" by voting for a person that the polls is telling them is going to lose.
Yes, I do believe that the poll results that come out close to election day could change the vote of some undecided voters. There are some people that even up to election day for one reason or another are still unsure of who they are going to vote for. If they see a high number of people siding with one candidate, they may believe that is the better candidate and vote for them.
As the days draw closer to election day, the majority of voters will have already made their decision regarding who they will cast their vote for. The campaign periods are long enough that the candidates have had plenty of time to present themselves and their platform. Additionally, the debates allow the two candidates to answer in-depth questions, and very often shows their character. At that point, the voters will have typically decided which way they will vote, so poll results close to election day usually remain about the same during the actual election.
I don't think polls have a huge effect on the actual election results. Most people decide who they're going to vote for long in advance of the election, and that doesn't change based on anything. The only slight effect polls could have is causing a few people to not vote because they assume that their candidate will win. That probably doesn't make a huge difference though, especially considering that most states are not very closely contested.