>>... But, if you use landlines, you exclude a considerable part of the potential electorate.
This is how we got those headlines "Dewey Wins in a Landslide!!" that Truman (the winner) was able to pose for photographers with. We've all seen the photo, yes?
That said, I find it interesting that maybe 30% of the Rebublicans voting in the SC primary did not vote for Trump. If 30% of the most-likely-to-vote people in your own party won't vote for you, they probably won't vote for the 'other' party's candidate, but they could easily stay home in the general.