Yes, poll results can have an impact on voters' opinions leading up to an election. The results of the poll may not have a significant impact on the way people vote, however, it can let people know whether or not a candidate is being taken seriously and should be paid attention to or ignored.
Yes, poll results alter voter opinions leading up the election. For example, right now if Hillary Clinton leads in the polls, will that mean that voters will be motivated to vote for or against her or stay home and not vote at all? The polls have great influence, even if we are unsure of the outcomes just yet.
If voters think that a candidate has no chance of winning they will often change their allegiance to a different candidate. This happens in politics all the time. The media is anti-Trump and they have an agenda to make him look like he is losing. Polls should not necessarily be believed and voters should make their decision based on what candidate they think will do the best job, not what the polls say.
We look at polls all the time, but I don't think they really alter voter behavior. They may change some people's opinions, but probably ultimately not their votes. In this election, there's not a lot of middle ground and I'm not sure how many undecided folks can be persuaded either way because of poll numbers.