How strong an impact does polling have on elections?

“Your Representative owes you, not only his industry, but his judgment; and he betrays, instead of serving you, if he sacrifices it to your opinion.” Edmund Burke

What is wrong with listening to the polls (and thus the people)?

If you want to vote for a candidate, say Rick Perry, you may decide that you are wasting your vote if he is polling low, and switch to your second choice.
If you really don’t want a candidate to win, say Mitt Romney, yet he is leading in the polls, you may decide to vote for the “Most Popular Candidate Who Isn’t Mitt Romney”, giving Rick Santorem (can I say that here?) a vote he wouldn’t have gotten otherwise.

@filmfann , nice analysis of how polling can affect things by getting people to vote for someone who is not necessarily their first choice.

Adding to @filmfann‘s answer, even if polls can cause just 1% of people to change their vote to the “Most Popular Candidate Who Isn’t Mitt Romney,” then that is a huge number. The voter turnout for the 2008 election was calculated to be 132,618,580. This would mean 1,326,185 more votes for Santorem that wouldn’t have happened otherwise.

Polling does have a large effect because people have short attention spans and will only hear what the canidate is presently saying rather than actually follow his previous acts for proof as to what he or she will do.
At the same time I also do wonder as to what people really do hear. I’ve listened to the same speeches as others and their interpetations are sometimes off or they have selective hearing.
There should be a test after speeches to see what the public actually does hear. LOL