A wire story from today's S-R reports on a Pew Research Center study of polling. It seems that some polls rely on contacting people with land-line telephones and therefore may be missing a disproportionately Democratic part of the population who don't have land lines, using cell phones instead.
Now rewind 62 years to the 1948 presidential election in which all the leading polls forecast Republican Thomas Dewey as the winner. After Harry Truman was elected rather handily, guess what they discovered. The pollsters had relied on telephone sampling when a lot of Democratic-leaning voters couldn't afford telephones.