Wiki:
http://en.wikipedia.org/wiki/United_States_presidential_election,_1948The Chicago Tribune, a pro-Republican newspaper, was so sure of Dewey's victory it printed “DEWEY DEFEATS TRUMAN” on election night as its headline for the following day. A famous photograph taken the next morning showed Truman grinning and holding up a copy of the newspaper. Part of the reason Truman's victory came as such a shock was because of as-yet uncorrected flaws in the emerging craft of public opinion polling. A political theory supported by many pollsters (and largely discredited by the 1948 election) held that voters had already decided who they would support by the time the political conventions ended during the summer, and that few voters were swayed by the campaigning done during the autumn. As a result many pollsters were so confident of Dewey's victory that they simply stopped polling voters weeks before the election, and thus missed a last-minute surge of support for the Democrats. It has been estimated that some 14% of Dewey's supporters -swayed by Truman's claims that an economic depression could return under GOP rule - switched to Truman in the final days before the election. After 1948, pollsters would survey voters until the day before the election: they would also announce their results on television, in real time, more or less.
Hope we see the mainstream media with egg all over their faces tomorrow.