According to Roger Stone, A Republican operative who has fought 9 Presidential campaigns, the way it works is that near the end of any election there are always about 20% undecideds and they are essentially followers, called "Bandwagon voters" and they tend to want to be on the winning side and want to be with the majority so that they win, just like sports fans support the best team etc
Near the end, the campaigns want to persuade those voters that they should back the candidate that the Establishment polls say is "going to win". But often the people know that the Establishment polls are not telling the true story and have a feeling on the street about who is really going to win. That is why the Establishment roll out their pop stars to try and win those voters back.
"Some Who Saw Change in Obama Find It Now in Donald Trump
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"Gary Kerns, 42, of Gettysburg, said he had been impressed with Mr. Trump for a long while and called himself a “bandwagon voter” — one who is drawn to the most popular candidates. In 2008, Mr. Kerns voted for Mr. Obama because he was moved by the charismatic young senator’s speeches.
“Obama was blazing hot,” he said. “There was momentum with him, and I got caught up with it. I loved it.”
Now Mr. Kerns, a registered Republican, sees Obama-like energy in Mr. Trump’s candidacy. “Let’s roll with the hot hand,” he said.
www.nytimes.com/2016/11/03/us/politics/obama-donald-trump-voting.html?_r=0