How strong an impact does polling have on elections?

I do not dispute the right of people to make polls and for the public to look at the results. I just wonder if there is a tendency to micromanage campaigns based on polling. There seems to be a tendency for politicians to tune their message to poll results rather than actually stand for something. I know politicians tend to do that even without polling, but the polls make it so much easier. How much different do you think election campaigns would be without polling?