I was having dinner with two other 50-ish adults, a Democrat and a Republican. The Dem said wistfully, “I sure wish they could find somebody to run other than Hillary.” After he had left the Rep said, “Do you have the slightest doubt that if Hillary is the candidate that he’ll vote for her?” I knew with all my being that he would. The Dem, who is an intelligent, college-educated, served-in-the-military professional simply could not bring himself to vote for a Republican.
As for doctoring the polls, on a small scale yes. On a national scale and across multiple pollsters, no. However, in political time the election is over fifty years from now, so much can change.
The fact that so many are running for the GOP nomination should tell you that they are a dying party, there is NO SINGLE candidate capable of galvanizing any significant part of the population.